Book of Revelations

Book of Revelations is an unlockable activated item.

Effects[]

Gives one Soul Heart.
Increases chance for a Devil/Angel Room door to open by 17.5% (35%) if held while killing the final Boss of the floor.
If the Book of Revelations is used at least once in a floor, a Harbinger will (have an increased chance to) replace the current floor's boss:

This item belongs to the Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.

Book of Virtues: Inner ring. The wisps become tiny crosses that do not fire tears, spawn a Harbinger familiar for the current room when destroyed.

Set bosses (Mom, Mom's Heart, It Lives, Isaac, ???, Satan, and The Lamb) cannot be replaced by the Book of Revelations.
Only the first boss of an XL floor is affected.
Book of Revelations won't replace the boss on double-sized Boss Rooms, like Double Trouble rooms or if the boss is Mr. Fred.
- Furthermore, if the Book of Revelations is used on the first floor of a Chapter with a Double Trouble room no Harbinger will spawn on the second floor, even if Book of Revelations is used again. It may be wiser to not use it on the first floor of Chapters 1-2 until you can see the Boss Room's shape.
If a Harbinger has already been fought in the current environment (e.g. if you activate the item in Basement II having fought Famine in Basement I) the boss will also not be replaced.
Keeping Book of Revelations until Chapter 4 (and more specifically, acquiring it on Chapter 1 by resetting the game until acquisition) is one of the methods to unlock achievements related to Super Meat Boy and Super Bandage Girl. The said method is explained in detail here.
This item can replace the boss even after Isaac initiates a battle with a normal boss and then teleports out before the fight is completed.
Alt path bosses will not be replaced with Harbingers, since the Harbingers cannot spawn on the alt path.
This item can replace The Bloat with a horseman in the Aprils Fool challenge.

The Book of Revelation (note the lack of an "s") is the final book of the Bible. It depicts the apocalypse through a vision shown to John of Patmos and includes the Four Horsemen and themes of Hell, Satan, and The Lamb. This explains why using it causes the next boss to be a horseman, as well as why it increases the chances of a Devil Room.

PC 3C6Z 880R (Treasure Room adjacent to spawn)

PC 9867 22FF (Treasure Room adjacent to spawn)

PC 42ZY EYNS (Treasure Room adjacent to spawn)

PC 6A7C BBKN (Treasure Room adjacent to spawn)

PC PS4 BQ7G DLKQ (Treasure Room adjacent to spawn)

PS4 G98Z MNSS (Treasure Room adjacent to spawn)

Vita RKV8 O883

3DS GKM3 XJJ0 (Treasure Room adjacent to spawn)

Switch V10Q 3WX1 (Starting item as Eden)

Switch DM91 QYEQ (First Treasure Room)