Death Ray Spell
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.
This video shows the location and the spell:
Mod creator: TrueChaoS
Based on a spell by Bethesda
Made using Bethsda's Creation Kit