At the sacrifice level on world 2, use the demon "Vincent" (Unlocked after reaching a certain pain score) and use one of his limbs to kill ??? by putting the limb on them, since Vincent burns constantly, the damage should kill the head. Once the head dies, you unlock "brains", which spawn in after the character's skull is destroyed(any character).
 "Mutilate-a-Doll 2"){kind=icon}
 "Drop Dead 3"){kind=icon}
 "mutate the labrat"){kind=icon}
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