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Effects

Grants one Black Heart.
(Removed in Repentance) Activates The Necronomicon effect upon taking damage if Isaac's total health (Red Hearts, Soul Hearts, and Black Hearts) is at one heart afterward.
(Added in Repentance) Deals 80 damage to everything in the room upon taking damage if Isaac's total health (Red Hearts, Soul Hearts, and Black Hearts) is at one heart afterward.
(Added in Repentance) Adds +40 damage to The Necronomicon and similar effects, such as Black Hearts.

(Added in Afterbirth)(Black Feather) Black Feather: Counts as an 'evil' item, granting a damage up.
(Added in Repentance) Blood Rights / Dry Baby / The Negative / (XIII - Death) XIII - Death: Since these items essentially activate The Necronomicon's effect, the damage boost is applied to these items as well.
(Added in Repentance)(A Missing Page) A Missing Page: The damage increase stacks, resulting in The Necronomicon and similar items dealing 120 damage.