D&Dtools1

Like the arcane spell meteor swarm, thunderswarm evokes blasts similar to those of the spell fireball, except that this spell creates balls of lightning.
When you cast it, either four large (2-foot-diameter) spheres or eight small (1-foot-diameter) spheres spring from your outstretched hand and streak in a straight line to the spot you designate.
Each sphere leaves a trail of sparks behind it.
Any creature in the straight-line path of these spheres is struck by each one and takes a total of 16d6 points of electricity damage (no save).
The spheres dissipate after dealing this damage.
If the spheres reach their destination, each bursts like an electrical fireball in a spread.
The patterns of their detonation and damage dealt to creatures in the area depend on the size of the spheres, as follows.
(See the meteor swarm spell description in the Player's Handbook for details on the patterns).
Large Spheres: Each large sphere has a 15-foot-radius spread and deals 4d8 points of electricity damage.
The four spheres explode with their points of origin forming either a large diamond or a large box pattern (your choice) around the spell's central point of origin.
The blasts are 20 feet apart along the sides of the pattern, creating overlapping areas of the spell's effect and exposing the center to all four blasts.
Small Spheres: Each small sphere has a 7 1/2-footradius
spread and deals 2d6 points of electricity damage.
These spheres explode with their points of origin forming either a box-within-a-diamond pattern or a diamond-within-a-box pattern around the spell's central point of origin.
Each of the pattern's outer sides measures 20 feet long.
All four areas overlap in the center of the pattern, and two or three areas overlap in various peripheral sections.
A creature caught in one of the blasts may attempt a Reflex save for half damage.
Creatures struck by multiple blasts must save against each blast separately.
Any creature that fails a save is stunned for 1d4 rounds in addition to taking full damage.