Mutilate I makes every landed blow do more to bring enemies to Near Death. It opens at level 3 for both twins.
Mutilate I sits at position 15 of 18 in the Combat tree. Unlocking Mutilate I takes 2 skill points. You need level 3 before you can buy it. Jacob and Evie each have their own skill tree, so you buy Mutilate I once for each twin.