Sphere of Decay
3rd level necromancy (Cleric, Sorcerer, Warlock, Wizard)
Casting Time: 1 action
Range: 60 feet / 10 feet sphere
Components: V, S, M (a chuck of rotting flesh or fruit)
Duration: Instantaneous
A tiny dot of blighted light is hurled from your fingertip to a place within range on the ground. Necromantic energy sips out from that point and fills a 10-foot by 10-foot cube, and decays any living matter inside it. Any creature within the cube takes 2d4 necrotic damage, can't regain hit points until the end of your next turn, and must make a Constitution saving throw. On a failure, a creature also becomes vulnerable to the next instance of damage it takes until the start of your next turn. On a successful save, the creature does not become vulnerable. A creature immune to necrotic damage is immune to this spell.
Using a Higher Level Spell Slot. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d4, and the size of the cube's dimensions increases by 5 feet for each spell slot level above 3rd.