Episode 23

Truth (真実) is the 23rd episode of the Ao no Exorcist anime overall and of the first season. It first aired on September 18, 2011.

Synopsis
Ernst's plan to destroy Gehenna backfires when Satan reveals to Yukio the truth behind his and his brother's origin. The brothers' mother, Yuri, decided to give birth to her 2 sons of demonic origin, Yukio and Rin, which led to her father, Ernst, giving an order to burn her and her sons. To protect Yuri, Satan possessed many human bodies, which were too weak to harness Satan's power and thus perished, including Ernst's, leading to the Blue Night before taking possession of his body.

Trivia

 * Ernst actually caused the Blue Night due to his bigotry.
 * Appropriately his malice comes full circle when he is pulled into Gahanna.
 * Yukio becomes Satan's vessel due to being his offspring; as Satan's power was already in Yukio from birth, he wouldn't be destroyed by possession.

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