Draining Kiss

A creature you can touch must make a Consitution saving through against your spell save DC. On a failed save, the target takes 5d10 psychic damage 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. 

At higher levels:

When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d10 for each slot level above 3rd.