When purchased at a Devil Room by ??? (or another character who has lost all Red Heart containers), this item effectively converts 3 Soul Hearts into 3 Black Hearts, increasing damage at no downside.
Synergies
Black Feather: Ceremonial Robes counts as an 'evil' item, granting a damage boost.
Prior to Repentance, the pickup quote was "Sin up."
Despite this, the item has always functioned exactly the same since its initial implementation in Rebirth. Previously, the damage increase went unmentioned in-game.
The change from 'sin up' to 'evil up' made the item consistent with others that grant Black Hearts — two examples being Match Book and Abaddon.
Black Hearts.
Devil Room by 
??? (or another character who has lost all Red Heart containers), this item effectively converts 3 
Soul Hearts into 3 
Black Feather: Ceremonial Robes counts as an 'evil' item, granting a damage boost.
Match Book and 
Abaddon.
The Binding of Isaac: Rebirth
