Energy Ebb

You point your finger and utter a foul incantation, releasing a black needle of crackling negative energy that suppresses the life force of living creatures. When you cast this spell, and as an action on each of your turns until the spell ends, you can target one creature you can see within range. If the target isn't undead, it must succeed on a Constitution saving throw
or take 4d8 necrotic damage and suffer one level of exhaustion. If the target is undead, you instead roll 4d8. The target gains half the total as temporary hit points.