Episode 20

Mask is the 20th episode of the Ao no Exorcist anime overall and of the first season. It first aired on August 28, 2011.

Synopsis
Rin and Yukio find that their friends from the monastery where they were raised were attacked by a masked individual who covered their entire bodies with an seemingly indestructible spider web. While pursuing the culprit, whose objective is to attack anyone related to the son of Satan to make him suffer, Yukio stumbles into Neuhaus who flees without further explanation. Despite having his sword sealed by the web, Rin insists to go after the masked one but is held back by Yukio and Shura, until he gets word that his friends, Suguro, Renzo and Konekomaru are being attacked too and rushes to help them. By managing to control his flames properly, Rin defeats the attacker, who flees thanks to Neuhaus who reveals her to be his wife, and safely releases his friends.

Errors

 * In a scene where Rin is attempting to save his friends, his sword is shown to be drawn, but in reality it is actually still sheathed.
 * Inscription in Polish seen in the monastery says "Nie popuszczę takiego, powiązanego z Szatanem" which is supposed to mean "I'll never forgive anyone related to Satan". It's a mistake since it actually means "I won't let go the one connected to Satan". A correct inscription should say "Nigdy nie przebaczę komukolwiek spokrewnionemu z Szatanem".

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