Dark Geyser

You select a point on the ground that you can see. A small gate to the negative plane of existence opens there, expelling pure negative energy in a 5-foot radius, 30-foot high cylinder of magical darkness. At the beginning of your next turn, the cylinder grows to a 30-foot radius and 60 feet high. At the beginning of your turn after that, it vanishes from existence.

Any living creature that’s in the area when the spell is cast, or ends its turn in it, takes 8d8 necrotic damage, or half damage with a successful Constitution saving throw. Undead creatures are immune to this effect; instead they heal 8d8 hit points if they end their turn in the area of effect.