Death's Grasp

You surround a 30 foot cube with a toxic black gas that sprouts from the cracks in the ground. Each creature within that cube that has fewer than 15 hit points dies. If a creature has more than 15 hit points, they must succeed on a Constitution saving throw take 1d10 necrotic damage, and is poisoned for one minute.

The necrotic damage increases by 1d10 every level. At the 10th level, the spell kills any creature within the cube who has fewer than 40 hit points. At the 15th level, the cube increases into a 60 foot cube.