"Necessarius" (必要悪の教会 (ネセサリウス) Nesesariusu?, lit. "Church of Necessary Evil") is the third episode of the first season of the Toaru Majutsu no Index anime series. It was first broadcast on October 18, 2008, and was directed and storyboarded by Tachibana Hideki and written by Akahoshi Masanao.