Level 7 Necromancy (Druid, Sorcerer, Warlock, Wizard)
Casting Time: Action
Range: 120 feet
Components: V, S, M (a fistful of grave-loam)
Duration: Concentration, up to 1 minute
You call up a devouring bloom of rot at a point you can see within range. Each creature in a 30-foot-radius Sphere centered on that point makes a Constitution saving throw, taking 10d8 Necrotic damage on a failed save or half as much damage on a successful one. A creature reduced to 0 Hit Points by this spell withers instantly to a drift of gray spores. The area then fills with rot-fog for the duration: it is Lightly Obscured and Difficult Terrain, and any creature that starts its turn there can't regain Hit Points until the start of its next turn.
Using a Higher-Level Spell Slot. The damage increases by 1d8 for each spell slot level above 7.