Uskius' Underdark Venom

With a word and swift motion, the target howls as the spell spreads through it. One creature in range is targeted. At the end of each of its turns for the spell duration, it must make a Constitution save: on a success it takes 2d6 acid damage, 2d6 poison damage, and 2d6 necrotic damage, and the effect ends on it; on a failure its maximum hit die is reduced by one. If the target is bloodied, it makes the save with disadvantage and takes spellcasting ability modifier acid, spellcasting ability modifier poison, and spellcasting ability modifier necrotic damage if it fails. If the spell reduces the target to 0 hit points it dies and its body is destroyed. This spell has no effects on undead or constructs.

If cast at higher levels, for each spell slot expended above 5th you can target an additional creature in range.