Abaddon

Abaddon is an unlockable passive item.

Effects[]

+1.5 damage.
+0.2 speed
- Also affects maximum speed (may never exceed 2.0).
Grants 6 Black Hearts.
Removes all Red Heart containers.
Converts all Red Heart containers into Black Hearts, and adds 2 additional Black Hearts.
Will not remove Bone Hearts, but will empty them.
Adds a chance to shoot fear tears.
This item belongs to the Leviathan set. Collecting three items from this set will transform Isaac into Leviathan.

The fear effect chance depends on the Luck stat, starting at 12.5% at 0 Luck, reaching 100% with 85 Luck.
- Dark Matter is significantly better at inflicting fear than Abaddon.
Does not remove Soul Hearts or Eternal Hearts.
As it removes all Red Heart Containers, other Deals with the Devil can be taken for "free" as long as Abaddon is taken last while still having at least 1 Container.
Has no negative trade-off when taken as ???, minus losing the opportunity of an Angel Room.
Can always be taken when offered as a 3 Soul Heart Deal. Even if Isaac only has one or two Soul Hearts, the Black Hearts will be granted after taking the item.
Will instantly kill the Keeper when picked up.
- Sets Keeper's Coin Hearts to 1.
Bethany will be left with half a Red Heart with her other Heart Containers removed.
Tainted Bethany will not lose stored Red Hearts, but can't gain more black hearts from this item as a result.

In the Hebrew Bible, the name Abaddon is used as a reference to a bottomless pit, often appearing alongside the place of the dead (which is known as Sheol), meaning the land of the dead. The actual Hebrew word Abaddon (אבדון - pronounced avadon) means "doom".
Abaddon is the Hebrew equivalent of Apollyon.
In the New Testament's Book of Revelation, an angel called Abaddon is written as the king of an army of locusts; his name is first transcribed in Greek (Revelation 9:11 – "whose name in Hebrew Abaddon").
Abaddon also shares its name with a level from Edmund's previous game Super Meat Boy.
Lil Abaddon resembles this item's costume, but its effects are closer to Maw of the Void.

PC SYE1 41CK (Red Chest on first floor teleports to Devil Room)

PC S97N 07DH (Curse Room adjacent to spawn teleports to Devil Room, also contains Lil Brimstone)

PC QJGK ZRFA (Red Chest in Curse Room adjacent to spawn teleports to Devil Room)

Bug!	If Keeper picks up Abaddon, it will remove all of his Coin Hearts but will not kill him until he picks up an item. This still applies if Abaddon is acquired via the Debug Console, but not if the item is picked up normally.

Bug!	PC If Tainted Jacob picks up Abaddon while in his ghost state, he will be unable to revert back and will die in one hit, even after changing floors.