Altars are a diversion that will get you new elements for your powers. In order to start it, you must use the appropriate power on the altar, then you will need to defeat a certain number of enemies in order to finish. The catch is that you have to use the element for the power you’ll be getting to kill the enemies (you can still use your weapons).

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Each Altar is assigned to a specific power and will reward you with a new element, but only after clearing waves of demons with that particular element first.