Death Blossom

You gain this spell on a roll of 29 or 79 on the Forbidden Spell Table.

You target one creature you can see within 150 feet of you.  That creature must succeed a Constitution save.  On a failed save, if the target has fewer than 200 hit points it is instantly slain, if the target has at least 200 hit points it takes 20d10 necrotic damage instead.  If the target is slain by this spell each creature within 15 feet of it must succeed on a Constitution save or take 20d10 necrotic damage.  This continues until no more creatures are slain by the spell or no more creatures are close enough to a slain creature to allow it to continue.