The invoker whispers the prayer: "Sweet Mother, sweet Mother, send your child unto me, for the sins of the unworthy must be baptized in blood and fear" . 2. Quest Initiation: "Innocence Lost"

Completing Aventus’s request is the first step to officially joining the faction:

Upon picking the lock and entering, the player finds Aventus performing the Black Sacrament, mistakenly believing the player is the summoned assassin.