Curse of the Beastlands

Like in the Beastlands, killing an awakened beast can lead to becoming the thing you killed. Over time, this risk expanded to Monsters. 

If you make the killing blow on an awakened creature of the Beast or Monster type, make a DC 15 Constitution and Charisma saving throws. On a failed con save, your body becomes that of the thing you have killed. On a failed cha throw, your mind becomes that of the thing you killed. If you fail, you have three days to pass the saving throw, or the effects are permanent.