Hogwarts schooling

Great plumes of cursed fire pour out from the tip of your wand, rampaging into shapes of beasts and incinerating anything in their path. Upon casting, the area of the fiendfyre is one 10-foot cube, with the number of 10-foot cubes doubling at the beginning of each of your turns. As long as concentration is maintained, you can arrange any newly added cubes 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. A creature takes the same damage when it enters the area for the first time on a turn or ends its turn there.