Blood leech

When You drop a creature or monster down to 0 hit points, you gain 1d6 of temporary hit points if you're at full health but you would instead heal that many points if you have lost hit points. later on at lvl 9 you would gain a 1d12 instead of a d6. At lvl 4 you can only gain up to 20 extra hit points. At lvl 9 you can gain up to 40 extra hit points.