Lightning Arc

Paragon Tier
Prerequisite: 11th level
Benefit: When you score a critical hit with a power that has the lightning keyword, you can choose to treat the attack as a normal hit instead of a critical hit. If you do, choose another target within 10 squares of the original target that was not already targeted or affected by the power. That target takes damage equal to the damage dealt to the original target.

Published in Player's Handbook, page(s) 204.