Unstoppable II protects your combo from the first blow that lands on you. It pairs well with Slayer, since keeping a long chain alive is how Slayer triggers.
Unstoppable II sits at position 9 of 18 in the Combat tree. Unlocking Unstoppable II takes 2 skill points. You need level 2 and Unstoppable I before you can buy it. Jacob and Evie each have their own skill tree, so you buy Unstoppable II once for each twin.