This is a much more powerful variant of the Fire Ball spell. 

A bead of white light, so bright that it makes the sun look dim, zooms from the caster's staff at the target. On impact, it explodes into a huge fireball 50 feet in width. Dealing massive damage to the target and anything surrounding it.

Roll 10d8 + 20 for damage. No Component Pouch nor Spellcasting Focus can be used to cast this spell. 

On a successful save the target takes 1/2 damage. On a failed save the target takes full damage. The target is knocked prone.