Cataclysm

You channel the power of death itself, attempting to end all life within range.  As soon as you begin casting this spell, the gods become aware of your transgressions and send their champions to stop you.  All manner of celestials, fey, and/or fiends (at the DM's discretion) appear in any number of unoccupied spaces within 120 feet of you, attempting to subdue you by any means necessary.  You cannot move or take any action, bonus action, or reaction without first interrupting your concentration on the casting of this spell.

Should you survive the onslaught and maintain concentration until the casting time is complete, every living thing but you within range instantly dies and disintegrates into black dust.  Undead creatures are immune to this effect.  For the next 1d4 x 100 years, the affected area becomes a desolate wasteland where nothing can grow.