Abaddon

Abaddon is an unlockable passive item.

Effects

+1.5 damage.
+0.2 speed and maximum speed (may never exceed 2.0).
Grants 6 black hearts.
Removes all red heart containers.
- Will not remove bone hearts, but will empty them.
Adds a chance to shoot fear tears.
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The chance to fire a fear tear depends on Isaac's luck stat, and goes up to 100% at 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 devil deals 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 ???, except the revoked chance of an angel room.
Likewise, The Lost gets a significant speed and damage increase.
Can always be taken when offered as a 3 soul heart devil deal. Even if the player only has one or two soul hearts, six black hearts will be granted after taking the item.

In the Hebrew Bible, the name Abaddon is used in 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 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.