Draining Kiss

You kiss a creature charmed by you or a willing creature. The target must make a DC  15 constitution saving throw or take 2d10 psychic damage on a failed save or half as much on a successful one. The target's hit point maximum is reduced by the number rolled and wont return until a long rest is taken. You gain hit points equal to the roll up to your max. The target dies if this reduces their hit points to 0.