The Necronomicon

Effect

Deals 40 damage to every enemy in the room.
(Added in Afterbirth †) This item belongs to the Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.

Many collectibles will produce effects similar to the Necronomicon, such as depleting a (Black Heart) Black Heart or blocking projectiles with Dry Baby.
The Necronomicon deals a considerable amount of damage to bosses that have multiple damageable parts, e.g. (Larry Jr.) Larry Jr. and (The Hollow) The Hollow.
This item also does high enough damage to instantly kill most non-boss enemies.

(Added in Repentance)(A Missing Page) A Missing Page / (Added in Repentance) Missing Page 2: Increases the damage from 40 to 80.
- A Missing Page + Missing Page 2: The damage increase stacks for a total of 120 damage.
(Added in Repentance) Book of Virtues: Spawns black wisps on the inner ring. The wisps deal 40 damage to all enemies in the room when destroyed. This is not boosted by BFFS!, but is boosted by (A Missing Page) A Missing Page and Missing Page 2.