Eventide

Sona Sitri dressed herself in a hurry.

She desperately wanted to be there for her friend and wished she had the luxury of comforting Rias, but she didn't have the time.

This was the first time either of them had lost a member of their Peerage, and Sona knew how much Rias considered them her family and how she saw herself as their supporter and guardian.

To have Eren be responsible... It gave the night's events an extra layer of horror.

But Sona couldn't be there for her.

With Issei... dead and Ophis missing. There was every likelihood that Eren could begin his attack on the Underworld in the next few minutes.

There was so much to do, her family to alert, citizens to evacuate or warn.

The Sitri territory was where the vast majority of the hospitals of the Underworld were located, and they needed every second they could gain to try and get the patients out.

Sona didn't know what sort of form the attack would come in. It could be a recreation of the Rumbling, combined with Senjutsu and backed by Ophis' power, as her aunt expected. It could be a targeted attack on Agares to get the Malabranche, a full-on invasion of the Underworld by the Chaos Brigade, or it could come in a completely unexpected manner.

No matter what, every second counted, and Sona had a duty as a King, as the Sitri Heiress, and as a leader to save every possible life she could.

Even if Sona's heart was heavy as lead and bile filled her throat, throwing herself into her work, into her duty, let her keep going.

The knock came right after Sona finished calling to wake up her Peerage. They'd gather for a group teleportation to the Underworld.

"Come in," Sona called.

"Sona," Mikasa Ackerman greeted her niece as she stepped through the door.

"Aunty," Sona replied in turn as she sent a quick email to the school. Nobody would see it for a few hours, as it was way too early in the morning, but since she was waiting for her Peerage anyway, she might as well inform them of everyone's, possibly long, leave of absence. "How are they doing?"

"...As well as can be expected." Poorly, in other words. Sona closed her eyes with a quick grimace, then got back to work. Similarly, her Contracts and those of her Peerage would receive a notice about a halt on current services for an indefinet period. "How are you?"

Sona paused her typing, taking a deep breath.

"...It doesn't feel real," Sona admitted. "We... weren't close. He was a pain in my side for most of his time at school, he and his two idiot friends. But..."

"...He's gone." Mikasa pulled her niece into a tight hug. Sona let herself be held by the older woman, clinging to a source of stability for a moment. "Death... It steals futures. What could have been. Hope, fears, dreams. All gone. Even those we don't love hurt us when they leave us."

"He was the first person I ever actually lost," Sona admitted quietly.

Sona had fought and killed before. She had clients pass away. Yet she had never had anyone she knew socially, someone she saw every day, die.

Even if she hadn't been friends with Issei Hyoudou and disapproved of his perverted antics, Sona had still admired parts of the boy. His dedication to his friends. His hard work ethic. Even his continued humanity despite becoming a devil.

Issei Hyoudou might have been a perverted idiot, but when he had gained power great enough to have people fight over it, he did not let it go to his head. Instead, he dedicated himself to becoming a wall to protect others.

One could not look at his stupid smile as he laughed with the devil children on the set of Oppai Dragon and see anything less than a simple, pure boy.

And now he was dead.

Killed by Eren.

"It doesn't feel real," Sona repeated, arms tightening around her aunt's waist. "I know you told us about... about your world. I know this had been his plan all along. But... I just..."

"...You never thought he'd go through with it," Mikasa's voice was all too empathetic. "Because he's Eren. Your friend. Because you love him."

Sona just held her aunt tighter as hidden tears flowed from her eyes.

In the coming hours and days, she'd need to be strong to be the leader her Peerage could look up to and rely on.

For just this moment, Sona put away her pride and allowed the tears to fall for a love lost.

"... I think Eren is sending us a message," Mikasa finally said after Sona's shaking stopped.

"What do you mean," Sona asked, drawing a deep breath and wiping her eyes to master herself. She couldn't afford to fall apart now. Not when so many people were counting on her.

"That he will go through with his plan, no matter what," Mikasa's eyes were hard. "He's prepared to do anything to achieve his aims. And that we need to stop him. I need to stop him."

"It's not your fault," Sona denied instantly. "He needed the Boosted Gear way before meeting any of us. And you don't need to be the one to... stop him, this time. The entire Underworld will be fighting him."

"...I left out a part in the story I told you all," Mikasa confessed. "After Eren broke out of the military jail with the Yeagerists and before the Rumbling, he found me and Armin. He... said some things to me that caused Armin to attack him. Eren beat him up. I just... stood there. Unable... No. Unwilling to do anything but watch. Even when my best friend was being attacked, I just... did nothing. Because it was Eren."

"And you loved him."

"I did. I do," Mikasa gave her niece a pained look. "I... was a slave to love. Eren said he hated that about me. That he hated me."

"He was lying." 

There was no doubt in Sona's voice as she rejected the very notion. She remembered the boy's tears earlier this year as he desperately confessed to wanting to live with the woman he loved.

"Maybe," Mikasa allowed, but she shook her head. "But he was acting on the knowledge his future self had given him. A part of his plan to reach the end he desired. The only reason he'd do that, take the time for it, is if having that meeting, antagonizing Armin and I... Hurting us... was necessary. Because if he hadn't done it and Eren hadn't said those things, I might not have been able to face him. Rumbling or not." 

"You think he killed Issei not just for the Boosted Gear but to tell you he wanted you to stop him again?"

"... All I know," Mikasa said softly, running a hand through Sona's hair. "Is that I wouldn't have been able to stop him if the last words he spoke to my face weren't that he hated me."

"I..." Sona didn't really have anything she could say at the moment. 

She wanted to believe Eren wouldn't kill a friend just to send a message, but... she also hadn't believed he'd kill Issei for power either. 

She could see it now, looking at the older woman. The guilt.

Mikasa might have had decades to confront what had happened, but she never got over having to kill the man she loved. Nor the fact that she could have done something earlier before so many people died.

And now Mikasa was seeing it all play out again.

Ultimately, Sona turned away from her aunt, unable to meet her gaze.

"Like I said," Sona cleared her throat to try and dispel the thickness in it. "You won't be fighting him alone. My sister, or one of the other Satans, might be the ones to face him."

"Maybe."

There was a beat of awkward silence, neither woman knowing where to go from there.

Eventually, Mikasa broke it.

"I'll go see Serafall. Give her everything I know of the situation," she said. "You can just head to your parents'."

"Thank you," Sona said, careful not to let the relief she felt show on her face. "My Peerage will be here in ten minutes, and we'll teleport to the manor. After that, we'll... We'll see."

Mikasa nodded, gave Sona one last hug, a quiet whisper to 'be careful,' and left.

Barely had the older woman closed the door, and before Sona could even get back to work, then another voice spoke up.

"Finally, nyaa. I thought she'd never leave."

"Kuroka!" Sona most certainly didn't yelp at the sudden voice and appearance of the former Stray. Anyone who said differently was a malicious actor aiming to sow dissent. "What are you doing here? Where's Azazel?"

"With my clone," Kuroka waived off the question as she approached the desk from where she had appeared in the corner of the room. "I have another with Shirone. We need to talk. Privately. And you cannot tell him, any of the Satans, or the bi... or her."

Surprise gave way to wariness, and Sona suddenly realized the precariousness of the situation.

Here she was, alone in the room with Eren's 'turncoat' lover less than half an hour after he gained the last parts he needed for his plan. Was Kuroka planning on jumping ships again and taking a hostage while at it?

Sona needed to make an excuse and leave. Now.

"Any sort of talk we have can wait till-"

"Eren can use the Founder."

Air left Sona's lungs in a gasp.

"...What?" She asked the severe-looking Nekoshou. "How?"

"Don't know how," Kuroka admitted. "Didn't know anything about 'royal bloodline' until she talked about it. All I know is that Eren has been able to see the future since before I met him. More than that, he has full access to the Path. Past, present, and future are all the same to him."

Strength left Sona as she sagged back into her chair.

"...He's already won."

Sona felt horror like never before as the surprise gave way to realization.

If Eren could use the Founder like he had in his world, he'd just choose a future he wanted and walk towards it. Every action they took, or could take, would be accounted for and part of the plan itself.

Eren had already lived through both 3 and 1 while they were stuck at 2.

"That's the thing," Kuroka hissed, leaning forward, her eyes wild. "If the world will become as Eren wants it to be, why is this the future he wants?"

From horror came confusion.

"What?" Sona asked in befuddlement. The suddenness of the catgirl's arrival and the shocking nature of her words finally overcoming her ability to roll with the punches.

Sona took a deep breath and looked at Kuroka. Really looked at her.

The older woman was... off. Her kimono was dishevelled, her hair was out of place, and her tails waved frantically behind her as if in a frenzy.

"Kuroka!" Sona said in as measured a tone as she could manage. "I am going to need you to explain. Too many things don't add up here. Aunty was certain he couldn't use the Founder. She would know."

"He couldn't," Kuroka nodded in agreement. Then, she shook her head. "He could use it before he came to Kuoh, but he couldn't while he was here, and he can use it again now."

Despite her question being technically answered, Sona was even more confused than before.

"Ok," Sona said, holding up a hand before Kuroka's face to stop her from saying anything else. Her other hand adjusted her glasses, the habitual motion calming her down. "Let's start with the basics. Why are you telling me this?"

"Because you're my best hope for helping Eren."

"...What do you mean?"

Sona didn't ask why Kuroka wanted to help Eren. She didn't even ask why the former Stray thought Sona would even try to help the boy who had just killed her friend's Rook.

Gather information before jumping to conclusions.

"None of us have told anyone about the Founder because Eren told us that if any of the leaders of the Three Factions learned of his ability, his plan would fail. Whether it's because of something they'll do or because the b- your aunt will learn of it from them, we don't know," Kuroka explained.

Before Sona could ask her next question, Kuroka answered.

"We don't want a 'Rumbling' situation either, and no idea what he... had done. But just because Vali and his team will fight him doesn't mean they want to betray him. Eren made certain promises to each of us based on the future he saw, and they are still waiting for theirs to be fulfilled."

"He promised to reunite you with Koneko, didn't he," Sona guessed, her mind making connections faster than ever before. "You never fought him, did you? You 'betraying' him was the plan from the start, so you could make a deal to clear your name."

"Why else do you think I'd work with a nine-year-old if not for Shirone," Kuroka gave the most minute of shrugs. "I was desperate."

"And Eren gained Senjustu from the deal," Sona continued. "One of the only supernatural abilities he could use and the most optimal to fit with a Titan's powerset... He's connected to The Paths. As soon as he activated the Founder, he experienced everything. He knew the exact steps to reach the desired end."

It was hard not to feel disheartened at the revelation.

What did Sona's preparations, plans, or desperate scrambling to save the Underworld matter if Eren had already won?

Even if Sona decided to tell her aunt right this moment, it would still be accounted for in Eren's future.

Was her race doomed to die at the hands of the boy she loved?

"...Why are you telling me this at all?" Sona asked again. "If my sister or the Satans can't learn of it, why not keep it a secret until he wins?"

"Because I need to do something. Because I need to protect Shirone, and Eren couldn't guarantee her safety, only mine," Kuroka's mouth curled in a grimace. "Eren... He sees only his own Path. There are no other Eldians in this world, not even your aunt. All he knows is what he, himself, experiences. Everything that is out of his sight, he is just as blind to. He has no idea why certain things happen, just that they do and the actions he has to take to reach the end he wants."

"And what end does he want?" Sona snapped. "What end is Eren chasing that is worth killing so many people? What future is worth the death of his friend?"

Even now, even after everything, Sona still couldn't bring herself to believe that their time with Eren had been a lie. That Eren hadn't cared about Issei, or Yuuto, or Koneko, or Akeno, or Rias, or... her.

"...I don't know," Kuroka admitted, tails drooping. "Eren... I know more about his Path than anyone, but he never told me the end. Just that I would survive and be reunited with Shirone."

"Then I can't help you," Sona said plainly. "Whether Eren wins or not, whatever future he's moving toward, I will do what I must. I will tell my aunt, my sister, and everyone else about the Founder. And then we will try and save as many lives as possible."

Sona Sitri was not a saint. She was not a hero or even someone with a particularly straight moral compass. She simply saw herself as a girl with a duty and a dream.

And to protect both, she'd fight for the Underworld.

Sona wanted there to be a way she could stop all this. She wanted there to be a future where they were all alive and happy and the imminent attack on her home by her friend.

But Issei Hyoudou's still cooling corpse proved there wasn't.

"I know three things." Kuroka's tails continued to thrash in agitation as she spoke. "One, that Eren didn't know why he'd do the things he'd do until he fled to Kuoh. He ran away from all of us, me included, to look for the gap in his ability to discover the answer he was missing, and he found it there. Two, he told me that the future he'd chase would be worse than The Rumbling, and he didn't want to do it. And three, he's smiling at the end."

...Worse than the Rumbling?

Worse than one and a half billion people dead?

And Eren was smiling?

"That's impossible," Sona denied simply. "Eren might not like devils or our society, but he wouldn't be happy even if he did kill all of us. He's not a monster." Then, another facet appeared in her mind. "There aren't even a billion devils alive. Even if he killed every member of the Three Factions, it still wouldn't be comparable to The Rumbling."

The devils had the largest population by far, thanks to reincarnated devils, but their numbers were still absolutely nowhere near that of humans.

Even if every high-class devil had a full Peerage, and each of their Peerages was also full, it still wouldn't dent the huge difference in population. High-class devils and above, whether in strength or rank, were the vast minority of the Underworld. 

Adjuka Beelzebub's evil pieces had never been designed for mass population conversion or the growth of armies. It was simply designed to turn a limited number of other races into devils, which would have children and expand the gene pool so devils wouldn't breed themselves out of existence.

Every reincarnated devil had a higher birthrate than pure-blooded devils, but it was still abysmal compared to humans. Even now, there were fewer devils than the total population of North America.

"That's what I've been struggling with this entire time, too," Kuroka admitted, her hand dancing across the cane in thought as she tapped it on the ground. "What changed? Only a few months ago, Eren struggled to figure out why he was smiling after everything. But now, he can kill his friend for that future? I need help figuring out what changed. Only then can I know what I must do to ensure he gets that smile he wants."

Eren wouldn't be able to smile if he did something worse than the Rumbing... Sona doubted he'd even be able to smile now, having killed Issei... so what future was he chasing? She could see why that would bug Kuroka.

Still...

"I still can't help you," Sona drew in a deep breath. "He could have lied to you about the future to get you to do what he needs. I don't have the Founder. I don't know the truth. Whether Eren smiles in the end or not, I have to do what I can now. Whether that leads to the future Eren wants or not, I have to chase the future I want."

"But, Sona," Kuroka's smile was all teeth. "Wouldn't a future where Eren can smile be one where he has to kill the least amount of people?"

...Damn it.

Satans, damn it!

This would be so easy if Eren really was a genocidal maniac, like Xenovia claimed. If his goal really was simply death and destruction, then it would be easy to just do the opposite of what he wanted.

But he wasn't.

Eren was just a boy who gained power. All his battles were for the same three reasons as every other sane person. Revenge, survival or ideals.

This world hadn't hurt Eren.

Eren was powerful enough to survive.

So, he was fighting for his ideals.

It wasn't hard to guess which ones.

He had made no secret how much he disliked devil society or why.

... If Eren was going to win in the end, it was Sona's duty to ensure it was as bloodless as possible. Hadn't she just said that?

Even if it meant aiding the destruction of her society.

"I want you to know that I hate you," Sona sighed in defeat as despair well up. "What do you want me to do?"

"I just need help figuring out what, exactly, changed between when I met him during the summer and when he left," Kuroka waived off her declaration of hatred easily. "Then I'll know what to do."

Sona didn't even comment on another layer of lies being pulled away. Kuroka had claimed to have only met Eren right before he left. She just filled it away for later as she had an answer right away.

"My aunt," Sona said simply. If Eren truly couldn't see anything he experienced in Kuoh with the Founder, then he wouldn't have memories of Mikasa, even with the Founder. "That is the biggest difference between when he left and the Peace Conference, which is when I imagine you actually met him for the first time."

"She's part of it," Kuroka agreed, but she kept tapping the cane on the ground in thought. "But not everything. I talked to him after they reunited, and he still wasn't sure why he'd do what he'd do. If anything, he felt worse because he'd hurt her again. I doubt that finally having sex is enough to make up for everything. No, I think on top of meeting your aunt, what changed was him entrusting his will to you all. But I thought that was for after he... you know, died. Now, I think it's something he needs others to do before he can reach the end he wants."

Kuroka had been spying on them since the Peace Conference, hadn't she? She had seen everything.

"...I can't think of anything else," Sona admitted, eye twitching at the reminder that her aunt had slept with her crush. She had been doing an excellent job ignoring that little incident, even if she had suspected as much. "You'd have better luck with Rias and her Peerage. I was very busy and barely saw him after summer."

"They're... not my biggest fans at the moment," Kuroka winced. Sona felt another pang of loss at the reminder that Issei Hyoudou was no longer with them. "...I honestly didn't think he'd go through with it. The boy... I liked him. He was funny. And I saw how Eren talked to him. I thought... I thought he'd lied about his need for the Boosted Gear. Or he'd have some crazy plan to get it without killing him... I don't know how he'll be able to smile after this."

Sona couldn't let the sadness overcome her. She had a job to do.

Her Peerage would be here any minute, then it would be off to the Underworld to deal with... whatever Eren's plan was. No matter what Kuroka thought, Sona would not let him destroy her home.

The only reason she entertained this conversation was because it gave her more information and gave her a good chance of saving lives.

"Your comrades don't have the answers, you can't talk to the Satans or my aunt, and Rias' Peerage won't talk to you. I am the only one you can come to try and figure out what happened to give Eren the conviction to kill one of his friends and possibly destroy the Underworld yet still be smiling at the end. Did I get that right?"

"More or less."

Sona closed her eyes, took a deep breath, and cast her mind back.

It was easy enough.

She had been thinking about those last weeks a lot recently. Wondering if she could have done something differently. If she could have stopped Eren.

If she hadn't been so busy and drowning herself in work, could Sona have stopped Eren from leaving?

It was easy, looking back, to see that Eren had known his time was limited.

What they had taken to be his awareness of his impending mortality had taken on a different meaning now that Sona knew he had seen the future.

Even if Eren couldn't see Kuoh, he would have seen his return to the Brigade and known when it would be.

Sona thought back to that last lesson, the one where Eren had tried to entrust her and Mikasa with his 'will.'

She had thought she had accomplished her promise by learning about his world from her aunt. She had heard about the Rumbling, his role in it, and the causes that led to it. Sona had internalized them as best she could, going so far as to write everything down so she could teach future generations about it as impartially as she could.

That had been what he'd been trying to do, after all. Ensure his world's lessons were not forgotten so they wouldn't repeat.

Or at least, that is what she had thought at the time.

Now, knowing he was planning on doing everything again, that conversation gained new depth and meaning.

Looking back on it now, Kuroka's words stirred something in her memory.

Eren had said he wouldn't have entrusted his will to Sona if Mikasa wasn't there, hadn't he? Only with her aunt to help guide her could Sona understand.

Did that just mean he needed Mikasa to tell her of their world, or did it mean something else?

Was Eren passing her a message when he told her to accomplish her dream? He had insisted that his Path had too many corpses and they shouldn't do what he did. Was she supposed to find another Path?

What was Sona's role in his plan? What was Kuroka? Or Mikasa's? Or Rias'? Or anyone's?

"You still with me, nyaa?" Kuroka waved her hand in front of Sona's face to try and get her attention.

Sona swatted it away.

"If you want me to help, you need to give me time to think," Sona said waspishly.

"Sorry," Kuroka shrugged. "But we really don't have much time. If we can't figure anything out by the time he starts his attack, then there's nothing we can do."

"...His attack," Sona mulled over the thought in her head as a realization hit her. "If he can use the Founder, he knows about the ambush."

"Sure," Kuroka nodded. "He probably would have even without it, to be honest. His sensing ability is still iffy compared to the rest of his Senjutsu, but even he would notice the gathering of High-class and Ultimate-class beings in Agreas."

"And he's still going to attack it!" Sona demanded in exasperation. 

Since it was the only place they knew for certain Eren would attack, a vital part of the plan the Satans had set up was a massive teleportation circle in Agreas to evacuate the civilians at a moment's notice and replace them with various combatants. If they did it quickly enough then Eren wouldn't have been able to sense anything.

"He has Ophis' power," Kuroka looked at Sona blankly, and the heiress winced at the reminder.

Right. Eren had the power of the Infinite Dragon and the Boosted Gear in his hands. He could plow through all opposition to get to the... Malabranche...and... unseal them?

"Wait," Sona suddenly said. "You said his sensing abilities are poor? Then how does he know where the lab with the Malabranche is? You claimed he sensed it while you were fleeing the Underworld."

"A lie to cover up the Founder," Kuroka waived the inquiry away. "Eren's abilities with Senjutsu come from his ability to practice without end in the Path. I actually have no idea how talented he is. He just cheats. But he can't practice sensing in there. He only learned sensing while in Kuoh because he had free time. Believe it or not, it's because of you."

Sona wanted to ask why it was because of her, but there was something else she wanted to clarify first.

"So he didn't sense where they were hidden before?"

"No, but it doesn't matter. Eren experiences past, present, and future all at once. Even if he hasn't sensed it in our past, only in our future, to him that has already happened."

"But have you seen the lab? The Malabranche?" Sona pressed.

"No," Kuroka denied. "Nobody has. Only Eren knows where it is. It's how he's kept Rizevim in line. He gets the other eleven only once Eren unseals Malacoda."

A shiver ran down Sona's spine, and the blood drained from her face.

"What is it," Kuroka asked, eyes narrowing. "You realized something, didn't you."

"...I don't think Eren's after the Malabranche at all," Sona eventually said. "Or at least, not them alone. Eren's after the ambushers. My sister, the Satans, their Peerages, and the other defenders."

"That... would make sense," Kuroka nodded thoughtfully. "Eren still needs a way to maintain his power in the long term. We weren't lying when we said Ophis' power-up is temporary, even if he stole most of the power, so he might still be after Malacoda. But if he could wipe out some of the strongest fighters in the Underworld at the same time, he would."

"But that can't be all." Sona's mind had now seized on a train of thought, and it wasn't letting go. "Eren might want to kill the powerful members of devil society, but those my sister works with, or recruits to defend the Underworld aren't going to be his main targets."

While not everyone the Satans worked with was necessarily good people, Sona knew her sister spent considerable effort in making certain her staff were people aligned with her ideals. Hells, Serafall only had two Peerage members, even after all these years, because she was incredibly picky. 

The other Satans might not be as strict in their choices of combatants. Still, Sona knew Sirzech's Peerage, at least, were also good people, and Sona had no reason to believe he wouldn't maintain his same standards when recruiting for the Ministry of Internal Affairs.

The Satans, their Peerages, and a handful of Ultimate-class devils would be those placed on Agreas to fight Eren.

And while Eren might not like them, they weren't the type of devils he hated either.

No, the type of devils he hated were those who were lazy, who lorded their positions and power over those weaker than them. Who used the Evil Pieces as slaves. Who were so deeply invested in the status quo that they fought against any change in the system that risked their power. Devils who opposed her sister's dreams for the Underworld...

...or who had been tricked into supporting Sona's dream.

Sona's heart seized in her chest as an idea came to her mind.

A terrible idea.

That terrible idea, almost against her will, formed into a calamitous plan. 

One who's simple utterance could see Sona executed.

"You went quiet again," Kuroka interrupted her downward spiral.

"... I don't know why Eren was smiling," Sona licked her suddenly very dry lips. "I can only guess. But I... I think I know what we... What I can do to lessen the number of deaths he'll cause to achieve his aim. That will probably be a big factor."

"Really," Kuroka asked eagerly, leaning forward and her hand tightening on the cane. "What is it?"

Sona shook her head.

She couldn't say. She wouldn't say.

Even the thought of it was... absurd. Heretical.

Evil.

She had no way to guarantee this would work or, if it did, that it would save lives.

And if it didn't? Then everything Sona had ever strived for, everything she had worked for, would come undone. Her dream would be ashes in the wind.

But if it did work... That was worse.

So much worse.

Sona Sitri would become the worst example of a devil in her society's history. A monster devil parents would tell their children about.

If she succeeded, and it ever got out, Sona wouldn't just be executed. She might very well end up in Cocytus herself, her Peerage executed, her family could lose everything, and even her sister might have to step down as a Satan.

The smart thing to do, the right thing to do, was to forget the idea entirely. To pretend Sona never thought of it, pretend this conversation never happened, and continue what she had been doing to aid evacuations and defence.

But now that Sona has the idea... Could she really do ignore it?

Could she ignore this terrible plan that could save her home and hundreds of millions of lives?

Knowing what she did now, knowing that Eren had the Founder and would succeed in some way no matter what she did, could Sona honestly live with herself if she didn't do everything in her power to stop the approaching calamity? Or at least mitigate it?

And if she didn't do this, would Eren really go through with his terrible plan to wipe devil society from the map?

Sona's mind went to Issei Hyoudou. An honest, perverted, hard-working devil. One who had fought his very hardest to accomplish his dream. 

While Sona might not have agreed with said dream, wasn't it just that sort of person her own dream was created to support? 

Hadn't she decided to dedicate her everything to a school where anyone willing to be taught, to learn and grow could come?

Eren was willing to kill his friend, the junior who had looked up to him, for the future he was chasing. Sona didn't know how he could smile after that, but knew that if Eren could kill Issei, he could march across the Underworld, crushing it into a sea of muddy blood if it meant reaching his goal.

Her aunt had said it, hadn't she?

Eren had been trying to send a message with Issei's death. One that underscored how far he was willing to go for his goals if he wasn't stopped.

Only now, to Sona, did the message take on a new meaning.

It hadn't been for Mikasa.

It had been for Sona.

But... could she do this?

As it had done for the last weeks, Sona's mind returned to that park, to that bench.

She remembered the conversation that had started it all vividly, that had started her on the Path that led to her developing a fondness for the blind boy.

She remembered the confession of a boy as guilt overwhelmed him. At the time, she believed he had been using a simple light novel as a metaphor for his time as a mercenary and having to make hard choices. 

After all, there was no way he had been in a time loop situation himself.

Now she knew the truth.

How many millions of timelines had he searched through to find the best ending? Why was this one the one he chose?

What was worth Issei Hyoudou's death?

Was it this decision, this plan?

If she was in his place, what would she have chosen?

Now, Sona had to make the hard call.

For that moment, Sona Sitri hated Eren Yeager. Hated him for being a stubborn, selfish man who relentlessly pursued his goals, no matter the cost. Hated him for sitting on that bench.

Hated him for putting Sona in this position.

Could she do this? Could she become a Devil if it meant saving those she cared about?

...Sona searched within herself and found the same answer as she had that day.

"Shinzou wo Sasegeyo."

"What did you say?" Kuroka asked, still looking eager as Sona came to a decision.

"Nothing," Sona said, her mind firmed even as something within her died. "I have a plan. One that might help Eren but has a good chance of saving the Underworld. But we need to start before he begins his attack, so we must move fast."

"What do I need to do," Kuroka asked seriously.

"How many clones can you make, and can they teleport?"

Sona relayed the plan as she began to write the first of dozens of letters.

Lord Zekram Bael,

In thanks for your continued support in my endeavours, and due to a fortuitous turn of fate, I wish to share with you and my other backers a once-in-a-millenia opportunity I have come across and that my sister and her colleagues desire to keep from your knowledge.

In exchange, all I ask is that you formalize your continued support for my school by signing the enclosed contract. I know this is most unusual and hasty, but time is tight before our window closes, and I urge you to act with all due haste...

Sona ended up having to send her Peerage ahead of her to the Underworld, promising to join them as soon as she was done.

She didn't want any of them to see her face as she wrote letter after letter.

If these letters ever became public knowledge, Sona Sitri would die, and her dream would die with her.

If they didn't... If Kuroka accomplished her side of the plan and destroyed them all before the day was done... If what Sona did ended up as just a rumour on the wind, she'd still be vilified by everyone.

As she wrote and wrote and wrote, every word consigning someone to their death and every letter another blow against her people, Sona could almost imagine Eren, on that first day they met, holding out a hand for her to shake.

Eren Yeager was truly the worst student Sona Sitri would ever have.

Yet, she made the deal.

A deal made to survive and make the world in their image.

And so, even though they were far apart and did not know what the other was doing, even though their feelings were never shared and no words were exchanged, a deal was signed.

The Devil shook hands with the Devil.