Berserk!

How do you unlock Berserk!?

In The Binding of Isaac, Berserk! is unlocked by earning the Delirium completion mark as Tainted Samson.

Isaac goes berserk for 5 seconds, greatly increasing his speed, tears, and damage, but restricting him to using a melee weapon instead of his tears. This item charges by dealing damage to enemies instead of clearing rooms.