Episode 22

Demon-hunting (悪魔狩り) is the 22nd episode of the Ao No Exorcist anime overall and of the first season. It first aired on September 11, 2011.

Synopsis
Ernst Frederik Egin takes over as the True Cross' newest leader and appoints Yukio Okumura as the new Paladin, as well as the Head of the Japanese Branch. Armed with special weapons that drain the blood of the Demons killed with them, the Exorcists take part in a widespread crusade to erase all Demons from Assiah. Rin Okumura and his friends start questioning why the Order decided to take such extreme measures, but Yukio is determined to accomplish his task as Ernst claims that it is all part of their plan to destroy Gehenna, and with the Demon World vanquished, both he and his brother would become completely human again. However, Ernst's real objective is revealed when he uses Rin's blood to open the Gate of Gehenna and bring Satan to the human world, Assiah.

Trivia

 * Mephisto appears to have limited foresight, given he escaped mere moments before the guards came in.
 * Another time where Mephisto shows childish traits is when he makes a bad drawing.

Manga & Anime Differences
This is an anime only episode, it does not appear in the manga.