Draining Kiss

You kiss a creature charmed by you or a willing creature. The target must make a DC  Constitution saving throw against this magic, taking 8d10 + Charisma Modifier psychic damage on a failed save, or half as much damage on a successful one. The target's hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

This spells damage increases by 1d10 at level 12 (9d10) 16 (10d10) and 20 (11d10)