Ovid's Tincture?

"No, no, no! This will not do!" said Daphne, a hint of panic creeping into her voice. "The old man must have gone senile in his last years. Why else would he have been creeping around the castle long after he was banished?"

"Hold on," said Oleandra, her brow frowning. "Obviously, Slytherin's creature is still here. So, if the old man could sneak back into the castle at any time, it stands to reason that his ideals never changed, since his arrangements are still in place. I don't know how the man grew up, but it's obvious the bloodline clans were dear to him, and that never changed."

"I suppose," whispered Daphne, wiping away a tear. "He did leave behind this potion recipe to help them."

A thought occurred to Oleandra. "So why keep it hidden here, then? He did mention it worked well in his letter, so why didn't he actually distribute it to the clans of his time?"

Neither Oleandra nor Daphne had an answer for that. Maybe the potion had an unexpected flaw?

In any case, Daphne had cheered up a little. Even though her entire world had been turned upside-down, the thought of still being superior to the vast majority of people did console her a little. 

Originally, pure-bloods had been superior to regular wizards (which included the Muggle-Born), but their bloodline factors had been diluted throughout the years. Factor in intermarrying with other pure-bloods with incompatible bloodlines, and copious amounts of inbreeding, hundreds of years later, their Fantastic Beast inheritance was more of a hindrance than anything, as members of the Sacred Twenty-Eight were about on the same level as non-nobles.

But the Bloodline Atavism Potion aimed to fix that problem, and restore the nobles to their former glory!

"We have to make that potion!" said Daphne excitedly, as she started daydreaming about being a powerful witch. "The recipe and the instructions don't seem that complicated."

"Hang on," said Oleandra. "I don't recognize one of the ingredients. Ovid's Tincture, what's that?"

Oleandra's question was interrupted by the sound of wood splintering and breaking loudly. The sister turned to look back; the entrance to the Scriptorium had finally managed to repair itself, expelling the wood that had been propping it open. They were sealed in.

Except they weren't. Oleandra had never let something as silly as an Anti-Disapparition Jinx stop her from leaving anywhere, and she also gathered there was another secret exit hidden somewhere in the room. 

"I found the secret exit," called out Daphne, right on cue.

Before leaving, Oleandra rummaged through her little pouch, bringing out a large pot full of dirt she had stolen from the Herbology classroom earlier on in the year when they had 'liberated' the Mandrakes. 

"Three cheers for the Undetectable Extension Charm!" she thought to herself, before planting a little sapling in the pot, and marking it with a rune. Now, they could return here at any time via tree-portation.

They were finally ready to leave. The secret exit brought them a corridor away from where they had entered the Scriptorium. Then, the girls rounded the corner back to the entrance, so as to get rid of the evidence. They hadn't learned the Vanishing Spell yet, so the best they could do was stuff whatever they could into their pouches and toss the rest into the flaming braziers that locked the entranceway.

"It's getting late, but I think Professor Snape should still be in his office. Shall we go ask him what our mystery ingredient is?" proposed Daphne.

Oleandra nodded, and they headed there in a hurry, where they found Snape about to close the door.

"Professor," she said breathlessly. "Do you know about Ovid's Tincture?"

"Where did you hear about that?" Professor Snape asked sharply.

Uh-oh. This probably meant this wasn't your average potion ingredient.

"We found a mention of it in an old letter in a book in Draco's home," Daphne lied as easily as she breathed!

With this, they were covering all their bases. They couldn't be asked to produce the letter, and if this was some sort of dangerous ingredient, the excuse of finding it in a school book wouldn't fly.

"Why are you only asking me this now, instead of at the beginning of the year?" asked Snape, not believing a word Daphne had just said.

Whoops. Maybe they hadn't covered ALL their bases.

"With all the commotion with the entrance ceremony, we just sort of forgot," explained Oleandra quickly.

"Ovid's Tincture is not something the likes of second years should know about," Snape said derisively. "Just know that this potion is incredibly dangerous and should not be messed around with. The changes it brings about are permanent."

And with those words, he turned and left, his robes billowing behind him as he went. Say what you will about the man, he knew how to make an exit!

"Do you think he enchanted his clothes to do that?" whispered Oleandra to her sister. "There's no wind."

But Daphne was in no laughing mood.

"It's not an ingredient, Ovid's Tincture is another potion!" she said darkly. "Who knows how complicated that one is to make! No wonder the recipe seemed so easy at first glance."

"A potion within a potion," said Oleandra, thinking. "It's bound to be in Moste Potente Potions, but it's in the Restricted Section of the library, and something tells me Professor Snape won't give us permission to read it."

"I don't think any teacher would sign our permission slip," said Daphne. "But hang on…"

The twins looked at each other.

"Do you think…?"

"Nobody would be that stupid…"

"Unless we buttered him up a little first?"

"Even then, there's no way a teacher would be this irresponsible."

"Professor Lockhart would sign anything if you asked him to, maybe we could trick him by asking for an autograph?"

"That could work!"

It was the perfect plan. Tomorrow, they had Defense Against the Dark Arts. Oleandra would disguise the permission slip as a photograph using Perthro. Then, Professor Lockhart would sign it, and they'd go down to the library to copy down the recipe. Then, they'd put the book back without signing it out, and none would be the wiser!