Amaterasu

A storm made of roaring flame appears in a location you choose within range. The area of the storm consists of a cone of flames 30 feet wide, 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 to save, or half as much damage on a successful one. It takes 10 charges off the sword to do this attack

When the true name of the blade is known the flames become black and instead do 10d10 of fire damage

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.