A book bound in leather made from human flesh, with an impression of a screaming face on the cover.
Upon activation, all enemies in the room will be damaged.
- Bloody Lust/Samson's Lock - All enemy projectiles in the current room are also destroyed upon use, which will count as "kills" for the extra damage effect.
- Triggering the effect of the trinket A Missing Page will also count towards unlocking The Necronomicon.
- This item deals significant damage during the Mom fight, as it hits all the doors and the leg, leaving Mom with little health left.
- This item works quite well against bosses with multiple segments (such as Larry Jr.) as every segment is damaged separately.
- The Necronomicon can damage off-screen enemies, which is possibly a bug.
- The Necronomicon is a fictional grimoire appearing in the stories of horror writer H. P. Lovecraft.
- It is believed to be bound by human skin, explaining the face on the cover.
- The book bears a strong resemblance to The Necronomicon in the Evil Dead series and the "Army of Darkness" movie.
- In the Collection, the Necronomicon is misspelled as "Necronomico".
- Using the Necronomicon on a Mask + Heart will only kill the 'masks', leaving the hearts alive, but one hit away from death.