FATALITY

Once per long rest you can select a creature you can see, declaring to the DM that you would like to "FINISH THEM", doing so makes your next attack an automatic Critical Hit.  If the creature is killed by this attack, you dispatch them in a particularly brutal fashion, making it so they can not be brought back to life (or unlife) by anything less than a Resurrection spell.

If this attack fails to finish the target creature, the Critical Hit is negated, dealing only normal attack damage.