Blue Exorcist: The Blue Night Saga

Rin Okumura's investigation into his birth and family history takes an unexpected turn when he's given a key by Mephisto Pheles that allows him to travel back in time 40 years. Using this key, Rin observes the life of his mother, Yuri Frederick Egin, as a child and an exorcist-in-training, particularly her interactions with Shirou Fujimoto, the man who adopted Rin and his twin brother Yukio. This journey through the past helps Rin uncover the truth about the events leading up to the Blue Night and his own birth. Along the way, he gains insight into the early appearances of Satan and the horrors at the True Cross Order's Section 13.