Blue Exorcist, Volume 19 (Blue Exorcist #84-88)

Mephisto Pheles has freed his brother Amaimon from imprisonment and set him loose on True Cross Academy to serve his own agenda. While the Exwires deal with Amaimon's surprise reappearance, Lewin Light, a.k.a. "Lightning," continues to investigate the mysterious Section 13 and its connection to the Blue Night event that seems to be the source of all the current troubles. Together they delve deep beneath True Cross Academy, shedding light on long-lost chambers shrouded in darkness and the terrible secrets that were buried there for a reason...