Found randomly in the Buried Tunnels or lower
In the middle of the room is an altar with a spike trap. Setting off the trap and bringing yourself to 1 or 1/2 health will spawn waves of enemies for you to defeat. Once the enemies are defeated, you're able to bind one of the passive items in your possession to yourself for future runs.
Each character has a separate instance of the altar and can have a different item bound to him/her.
Binding an item will only bind it to the current character.