Episode 24

Satan's Spawn (の) is the 24th episode of the Ao no Exorcist anime overall and of the first season. It first aired on September 25, 2011.

Synopsis
Rin and his companions manage to flee from Satan who starts destroying the True Cross Academy with his fiery blue flames. Knowing that Satan still stands beside the Gate because Rin has the power to destroy it, Shura comes up with a plan to distract him to give Rin an opening to do it, while the other students help evacuate the school. Their plan fails when Shiemi intervenes trying to dissuade Yukio with no success, but Rin manages to have his brother regain conscience and expel Satan from his body. However, the gate keeps getting stronger and it will not be long for Gehenna and Assiah to merge into one single realm.

Trivia

 * Despite his malevolent personality, Satan is actually trying to fulfill a promise to Yuri to create a new world.
 * It appears this inspired the illuminati's plans in the manga.
 * Satan seems to have a perverted sense of humor, given he taunts Shura about her breasts.

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