"Creates strong electric potential difference between muzzle and nearest target."
The Lightning Gun fires a purple electrical discharge, arcing to one adjacent target. This is a very convenient weapon, since Trace only needs to aim it in the general direction of an enemy for the Lightning Gun to find its target and deal damage.
The Lightning Gun is located in the lower-mid section of Eribu, past some level 3 corruption in the upper-left section of a vertical shaft filled with Red Nautili, Spiru, and a Green Cephalopod. Trace must use the Address Bomb to access the tunnel it's located in, then fight past 5 Purple Sentries.