The Sun

You summon a massive ball of fire above your head, and then shoots to a point you choose within range with a HUGE roar into a fiery explosion. Each creature in a 40-foot-radius Sphere centered on that point makes a Dexterity saving throw, taking 20d12 Fire damage on a failed save or half as much damage on a successful one.

Flammable objects in the area that aren’t being worn or carried start burning or simply incinerate. (DM discretion)

You can only cast this spell once every long rest.

Using a Higher-Level Spell Slot. The damage increases by 1d12 for every other character level above 9. There's a surprise at level 18...