The Nail

How do you unlock The Nail?

In The Binding of Isaac, The Nail is unlocked by earning the Boss Rush completion mark as Azazel (Character).

[Before Repentance] Gives one Soul Heart. For the current room; +0.7 damage, -0.18 speed, Isaac breaks rocks and deals contact damage. / [Repentance] Gives half a Black Heart. For the current room; +2 damage, -0.18 speed, Isaac breaks rocks and deals contact damage.