Death Coil

With  a chilling sound, you hurl an animated skull from the tip of your sword that feeds from your enemies despair!

Make ranged spell attack agains the target. On a hit the target takes 1d6 necrotic damage. The target must succeed on a (DC) constitution saving throw or its 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 the effect reduces its hit point maximum to 0.

A celestial being hit by this attack takes an extra  2d6 necrotic damage.

When you cast this spell using a spell slot of 2nd level or higher, the damage increases by 1d6 necrotic damage and 2d6 extra necrotic damage on celestial beings for each slot level above 1st.