Lightning gun

The lightning gun is a new energy weapon which appears in the multiplayer and SnapMap modes of the Doom game. It fires a constant beam of directed electricity (presumably via technology). It can attack either directly, or via splash damage-inducing electrical discharges targeted at the floor. The Night Sentinel troops in The Ancient Gods, Part Two carry similar weapons, although they aren't shown firing them.

"Short range energy weapon that fires a constant stream of electricity."

- SnapMap description

Pre-release
In The Art of DOOM, the concept for the lightning gun is called the ARC cannon. It is stated to have been conceived by the UAC as a counterpart to the plasma rifle designed at the Advanced Research Complex and based on research into Argent conductivity. It utilizes modulation of the plasma current to achieve a sustained linear stream of discharge which is destructive to biological forms. Its production was stated to be limited, perhaps meant as a nod to its eventual multiplayer-only status.