mage spell "Fighter Envy" or "Wizard Vanity"

When targets deal less damage than their weapon or barehanded attack is capable of (or for NPC's, their level) the difference is added to their health.
e.g. a PC with a max damage 36 weapon hits for 30, thereby gaining 6 health
e.g. a PC hits barehanded for 1, thereby gaining 3 health
e.g. a level 19 NPC hits for 12, thereby gaining 7 health