Dark souls puella magi

You are a [magical girl/undead] who cannot easily be permanently killed by physical means, but you must defeat [witches/hollows] in order to obtain [grief seeds/humanity], in order to prevent your soul from being tarnished and succumbing to despair. Those [magical girls/undead] who fall to despair become [witches/hollows] and lose their former selves, becoming what they once fought. This cycle is necessary because of the effects that entropy has on the world.