Flame streaks from your finger to a point of choice within range, flashing into a white hot explosion of flame.  Each creature in a 10-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 16d6 fire damage on a failed save, or half as much damage on a successful one.

The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.

After years of study and careful observations Yasuko Koehlanna has modified her fireball casting to increase damage intensity by reducing the area of effect.  Due to the advanced nature of this spell it may only be learned by level 19 wizard or above.  This spell contains so much energy that it may not be contained or stored in any vessel.  Trying to store this spell in any object will result in an instant detonation of the spell, the spellcaster trying to store the spell and anyone in a 10-foot radius sphere centered on the spell caster will take full damage of the spell at the level attempted and anyone between 10-foot and 20-foot radius of the spellcaster will take half damage at the level attempted.    

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 2d6 for each slot level above 3rd.