Wondrous Item, varies
Craft recipe:Empty magical enforced iron ball(cost 1000gp)
A spellcaster of 15th lvl. must cast 200 magic missile spells into the iron ball(aprox. 6 Hours to prepare)than it becomes armed.When its armed spellcaster must hold concetration on the bomb or it will explode(after 36 seconds after losing concetration).Spellcaster can set the detonation time up to 1 hour,after the time is elapse the Annihilation Sphere Bomb deal 400 force damage in 120 ft. sphere radius.
Colour of magic missile is up to spellcaster who created it.