Consumptive Flames

You unleash a torrent of flame, immolating everything in its path as it spreads.
Persistent flames take form in a 60-foot cone.  For each round you concentrate on this spell, the cone increases by an additional 60 feet. If a creature is within the area of the cone when the flames first reach it, or starts its turn within the area, it must succeed a Dexterity saving throw. A target takes 12d6 fire damage on a failed save, or half as much damage on a successful one. A creature cannot receive damage from this spell more than once each round. The fire spreads around corners and ignites flammable objects in the area that aren't being worn or carried.

If you concentrate for the full duration of the spell, you may continue concentrating to increase the effect's range, though sacrificing your own vitality to do so. For every round you concentrate in this way your current hit points and hit point maximum are both reduced by 10. This reduction can't be lessened in any way, and the reduction to your hit point maximum lasts until you finish a long rest.