Location

The spell tome is located on the ground by the end of the right side of the gate bridge in Whiterun (beside the Blacksmith).

~ Video below.

What it does

The spell tome teaches you a modified "Sparks" spell which has just short of 1,000,000 damage. This means that it can kill literally everything that is within range.

Note: The spell DOES have quite limited range, as with the Sparks spell, and cannot kill essential people (like children) unless you have a mod that makes them killable.

Video

This video shows the location and the spell:
http://youtu.be/aZjmP1PCAbk

Credits

Mod creator: TrueChaoS
Based on a spell by Bethesda
Made using Bethsda's Creation Kit