You create a wall of fire on a solid surface within range. You can make the wall up to 60 feet long, 20 feet high, and 5 feet thick, or any shape with an area of 300 sq feet or less.

When the wall appears, each creature within its area must make a Dexterity saving throw. On a failed save, a creature takes 5d8 fire damage, or half as much damage on a successful save.

This wall deals 5d8 fire damage to each creature that ends its turn within 5 feet of the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there.

For each round after the initial round that this spell is active, the caster can increase the number of spaces affected by this spell by up to a factor of two. For instance, if there are 12 affected spaces, the caster can add an additional 12 spaces to the blaze. Each new spaces must be touching a space that is already ablaze from the previous round. If the caster is not conscious and controlling the blaze, the number of affected spaces will double each turn in this way randomly. 

The blaze only stops if the caster consciously ends the spell. 

At Higher Levels. When you cast this spell using a spell slot of 7th level or higher, the damage increases by 2d8 for each slot level above 6th.