The title appears to refer to a specific adult-oriented visual novel or indie game titled Saint Elna and the Book of Depravity
The Book of Depravity is not a spellbook. It is a . Its central tenet, as later preached by Saint Elna (before her execution), is this: saint elna and the book of depravity
"She read what angels fear to whisper. And she found God laughing." The title appears to refer to a specific
'I, Elna, now sin with full consent. Therefore, I am finally free to love without performance.' And she found God laughing
"To reject depravity is to lie about the shape of your soul. A saint who has never envied is not holy—they are hollow. A hero who has never lusted for ruin is not brave—they are a puppet. Depravity is not the opposite of virtue. It is the soil from which virtue grows."