Draining Kiss

You kiss and drain the life energy from a creature that is charmed by you or unconscious. The target must make a constitution saving throw. On a success, it takes 2d8 psychic damage, or half as much damage on a successful one. You regain hit points equal to the damage taken by your target. In addition, the target's hit point maximum is reduced by an amount equal to the damage taken. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0 or below. 

If you cast this spell with a spell slot of 2rd level or higher, the damage increases by 1d8.