The Chainlightning Gun was a firearm that was promised for the full version of Quake in the readme of Qtest, along with the Thunderbolt. However, it was cut from the final game. It was going to zap lightning into the player aimed at. That player would chain lightning to any player that they see, continuing until no one was left in view. It would have used Cells just like the normal Lightning Gun.
The only evidence of its existence besides the readme is a HUD image. It is similar to the Thunderbolt HUD image, but is red instead of yellow.