Level
7th
Casting Time
1 Action
Range/Area
150 ft.
Components
V, S
Duration
Instantaneous
School
Evocation
Attack/Save
DEX Save
Damage/Effect
Fire
A storm made up of sheets of roaring flame appears in a location you choose within range. The area of the storm consists of up to ten 10-foot cubes, which you can arrange as you wish. Each cube must have at least one face adjacent to the face of another cube. Each creature in the area must make a Dexterity saving throw. It takes 7d10 fire damage on a failed save, or half as much damage on a successful one.
The fire damages objects in the area and ignites flammable objects that aren't being worn or carried. If you choose, plant life in the area is unaffected by this spell.
The spell does 7d10 damage. One cube touching a creature? It takes 7d10 damage. 7 cubes touching a creature? It takes 7d10 damage.
This is overpowered.
The RAW area argument is fine just as long as you stop applying any logical thought at RAW and suspend disbelief. But the reason this is a question everywhere is because the RAW doesn't make sense. If I rake a machine gun across a squad of soldiers they'll all eat a few rounds of 7.62 and take their 7d10 damage. If I rake the same machine gun at King Kong, he takes ALL the damage. If I cast a firestorm that does 7d10 in anywhere of 10 discrete 10x10 packets at a huge dragon that's 10x100, he eats 70d10 damage. Not by RAW, only by logic. Your table doesn't have to play by logic, but mine does. It's not RAW, it's maybe not even RAI, but it's a lot more fun.