I like that he, (unlike other authors), knew that after reincarnation your lover doesn't love you unless it was established before the time frame of reincarnation.
I think it was a nightmare from the past, with the stacks of bodies in a throne room with the ghost story from 300 years ago. but the beginning and end were his own.
Ch 51 Chapter 9.5 - Two attendants, mana depletion, and headpats (part 2) + nightmare
Even though the dark magic is frowned upon, I think it fits well as he is becoming a princesses attendant and can literally watch from the shadows to make sure no harm comes to her.
Ch 24 Chapter 6 - awakening and Awakening (part 5)