Slayer II chains Near Death states from one enemy to the next, which sets up big Multi-Finishers in group fights. You need level 3 and Slayer I.
Slayer II sits at position 11 of 18 in the Combat tree. Unlocking Slayer II takes 2 skill points. You need level 3 and Slayer I before you can buy it. Jacob and Evie each have their own skill tree, so you buy Slayer II once for each twin.