All-Consuming Blackfire

A roar of violent black flames erupt from a targeted point within range and start spreading. The flames ignite flammable objects, items, plants, and creatures in a 15 by 15 ft. area centered around the targeted point. They will then grow outwards by 5 ft. per round and will continue to do so until they are either put out by suffocation or complete submergence in water, - OR - dispelled by a dispel magic spell, or if the caster expends another 9th level spell slot, - OR - until they consume all flammable matter in the area.

If a creature is the target of the flames point of eruption or either start their turn within- or move into an area on fire, they must succeed on a Dexterity saving throw with disadvantage or catch fire. The flames burn for 2d10 of Fire Damage per round, and increases by 1d10 for every round. A creature ablaze will spread the flames outwards by 5 ft. at the end of its turn.

A creature immune to fire damage doesn't catch fire at all and can move through the flames unharmed.

At Higher Levels: When you cast this spell using a spell slot of 8th level or higher, the burning damage increases by an extra 1d10 per round for each slot level above 7th.