It flips traditional tropes of the "pure nun" on their head.

: Characters who appear pious but are actually trained monster hunters or assassins.

The long-running series A Certain Magical Index offers a very famous example of this theme. The character Index is a young nun from the Church of England whose "secret" is that she has committed the 103,000 forbidden magical texts of the Index-Librorum-Prohibitorum to memory, effectively becoming a living, walking library of dangerous knowledge. This secret puts her life in constant danger and drives much of the story's conflict. While different in tone, its central "secret" makes it a key entry in the genre.

Keywords used: The Nun 39's Secret Manga, nun manga horror, chapter 39 nun secret, gothic manga nuns, psychological horror manga.