Abaddon

How do you unlock Abaddon?

In The Binding of Isaac, Abaddon is unlocked by earning the ??? completion mark as Azazel (Character).

[Before Repentance] +1.5 damage, +0.2 speed. Removes all Red Heart containers, grants six Black Hearts, and adds a chance to shoot fear tears. / [Repentance] +1.5 damage, +0.2 speed. Grants two Black Hearts, converts all Red Heart Containers into Black Hearts, and adds a chance to shoot fear tears.