Notched Axe

How do you unlock Notched Axe?

[Before Repentance] Isaac destroys obstacles and opens Secret Rooms by walking into them, but cannot shoot or pick up items until it's cancelled. / [Repentance] Toggles between tears and the pickaxe, which can be swung to attack and break obstacles. Using it depletes the charge, and it can't be used until next floor once the charge runs out.