Aura of Undeath

You touch a point on the ground, and a dome of withering death spreads out in a 30 ft. radius and remains there for the duration. A living creature that enters the aura or starts its turn within it must make a Constitution saving throw. On a failure, the creature takes 8d6 necrotic damage, and its hitpoint maximum is reduced by the same amount until a long rest. On a success, a creature takes half as much necrotic damage and its hitpoint maximum is not reduced. Moreover, while in the area, a creature cannot regain hitpoints, either through magical or mundane healing. A creature whose hitpoint maximum is reduced to 0 or lower dies, becoming a zombie under the caster's control. Undead and constructs are not affected within this area.

Using an 8th-level spell increases the duration to 24 hours, and a 9th-level spell causes the aura to remain indefinitely until dispelled.