Slayer I rewards long combos: hit x10 and the next strike leaves your target at Near Death. It costs one point after Sequence 1 and works well with Multi-Finisher.
Slayer I sits at position 10 of 18 in the Combat tree. Unlocking Slayer I takes 1 skill point. You need to complete Sequence 1 before you can buy it. Jacob and Evie each have their own skill tree, so you buy Slayer I once for each twin.