Circle of Madness

As you swallow your bite of the tongue you see inside Cthulhu's mind itself, causing a blast of dark energy driving all those inside its radius mad except for the caster. If consumed by a normal being, their mind will be driven to madness permanently, but the spell will be all the more powerful.

A sphere of negative energy ripples out in a 15-foot-radius sphere from a point within range. Each creature in that area must make a Constitution saving throw. A target takes 8d10 necrotic damage on a failed save, or half as much damage on a successful one.