Bigby's Besieging Bolt

Evocation (Force)

Level: Sor/Wiz 6
Components: V, S, F
Casting Time: 1 standard action
Range: Long (400 ft. + 40 ft./levels)
Effect: One siege bolt per 2 caster levels
Duration: Concentration
Saving Throw: None
Spell Resistance: Yes

With this spell you are able to fire one magic siege bolt for every two caster levels. The bolts appear next to you and fling outward to attack as directed by you. You may launch one bolt each round for as long as you maintain concentration (although the spell ends once you have launched one bolt for every two caster levels). The bolts strike their target unerringly, and all damage inflicted is considered force damage. The type of siege missile the bolt mimics depends on your caster level.

Level Siege Bolt Damage 

11th Light catapult 4d6 

13th Heavy catapult 6d6 

15th Trebuchet 8d6 

Material Component

A small stone sprinkled with diamond dust (worth 100 gold pieces).