Cataclysm

This Spell, By most, have been forgotten to the lengths of time in which it hasn't been used. This spell tack years upon years to master, and upon casting it, it will be the last thing the caster will do. As a last ditch effort, anyone within a 20,00 square ft burns with the fires from the lowest pits of hell. Creatures within this square must succeed on a dexterity saving throw or take the full brunt of 900d20 fire damage.