Stray Blade

This will be unlocked fairly early in the game and depending on if you die a lot or not, you could get this even earlier. Some areas have respawning enemies, but not all, and the run back will have you kill them again or flee past them.

Anyways, once you reach level 6 you will have enough skill points for this to unlock. Just go in your skill tree and find a skill you like and spend the points.