You summon a column of dark, crackling energy swirling in a 60-foot-tall cylinder with a 30-foot radius centered on a point you choose within range that lasts until the spell ends. The area is filled with violet dim light and it must be oriented vertically.

Until the spell ends, on each of your turns, you can use an action to strengthen the spell with your magic. If you do, the radius of the area grows by 10 feet for the duration. This cannot extend the radius beyond 60 feet.

At the end of each of your turns while the spell lasts, dark magic crackles, and each creature in the area must make a Constitution saving throw. On a failed saving throw, a creature takes 3d8 necrotic damage, or 6d8 necrotic damage if you cast the spell or used your action to strengthen it this turn. On a successful saving throw, a creature takes half damage.

Undead and constructs are immune to this spell.