Ao no Exorcist: Yosuga-hen

After Yukio parts ways with Rin and heads toward the Illuminati in search of strength and truth, Rin realizes that chasing his brother will mean little while the secret of their birth remains hidden from him. Mephisto gives him a key leading forty years into the past, where Rin follows his mother Yuri Egin from childhood and witnesses her bond with the young Shiro Fujimoto long before he became the man who raised the twins.

From the experiments of Section 13 to Satan’s first appearances in the human world, Rin watches the events no one ever told him about gather piece by piece toward the Blue Night. As the day of his and Yukio’s birth approaches, the question he avoided for years becomes impossible to escape: what truly happened between Yuri and Satan, and what does it mean for Rin to carry the result of that tragedy within himself?