Azareck's Debilitating Word

You speak a word of arcane power to mutilate you opponent. The target must make 5 consecutive Constitution saving throws. For every failed save the target loses a limb or body part of your choice, with the head being the last thing to go always. If it has no limbs, it loses something of similar importance, each time taking 2d10 necrotic damage on a fail, and half as much on a success.  These cannot be regenerated - except through Greater Restoration or something similar and then casting the Regenerate spell afterwards. If the creature loses their head they die outright, unless they can survive without a head. Undead and Constructs have advantage on the save and Celestials have disadvantage.