Cain's Other Eye

How do you unlock Cain's Other Eye?

In The Binding of Isaac, Cain's Other Eye is unlocked by earning the Boss Rush completion mark as Cain (Character).

[Before Repentance] Spawns a familiar that slowly floats around the room on a zig-zag pattern and shoots normal tears in the same direction as Isaac. / [Repentance] Spawns a familiar that follows Isaac and fires tears in random cardinal directions that copy Isaac's tears and tear effects and scale with damage.