Combat Stamina heals you whenever you land a Multi-Kill, scaling with how many enemies drop. At six points and level 6 it is the most expensive Combat skill.
Combat Stamina sits at position 18 of 18 in the Combat tree. Unlocking Combat Stamina takes 6 skill points. You need level 6 before you can buy it. Jacob and Evie each have their own skill tree, so you buy Combat Stamina once for each twin.
Adrenaline
Raises how fast your health regenerates.
Counter
Press the counter button as an enemy swings to turn his attack back on him.
Counter Boost
Counters get a wider timing window and deal more damage.
Execute I
Stun an enemy with Stun Attack, then use a Tool Combo on him to execute him outright.