Episode 21

The Secret Garden (秘密の花園) is the 21st episode of the Ao no Exorcist anime overall and of the first season. It first aired on September 4, 2011.

Synopsis
Shiemi in the Exorcist Shop finds a woman lying unconscious in their garden. Meanwhile, Yukio meets Ernst Frederik Egin, who claims to be his grandfather, and reveals the truth about their Mother who was burnt alive by the Vatican proclaiming her to be a witch. He takes Yukio to a secret anti-demon Dragoon Laboratory and suggests Yukio to join him now that Shiro is dead and Mephisto has been arrested. Prof. Neuhaus is being interrogated in the Academy's Great Cell by Shura where he tells everyone (including Rin) that his wife, (Masked Man) Michelle, is nothing but a corpse with a certain demon in her body, intending to kill Satan but she was not resurrected from death by him. Rin finds out the woman in the garden to be Neuhaus' wife but Shiemi stops him from killing her. Paladin Angel appears in the garden to kill the demon but confronts Rin. Later, Michelle dies protecting Rin from the Order.

Trivia

 * Ernst is heavily implied to have caused these events out of his monomaniac hatred of demons. The chaos allowed him to usurp the Grigori and Mephisto's control of the Order.
 * Obsesseion like his leads to gradual declining of one's sanity.
 * The fate of Igor is up to the viewer as its not directly stated if the vines serve as the grave for him and Michelle or simply a barrier to let him escape with her body.

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