Lightning Cannon (Bureau)

A weapon that uses Earth's technology and knowledge of ballistics, combined with the Outsider's Elerium technology, to produce a weapon with unparalleled damage output.

After the Spire mission, you can help test-- and blow up-- the prototype.

Endgame Spoilers:

It can be found in the second-to-last mission, when XCOM's base is under attack. When approaching the room it is in, there is an open door that a Scientist manages to step through before being shot by the Infiltrator. Past the door, you'll find yourself on a balcony overlooking a round device with electricity arching off of it. On the far opposite corner, on the floor behind a desk, you will find the Lightning Cannon.

It is acquired in the Hangar 6 DLC in the lookout station following the completing the Evacuation mission.

Naturally, as it's such a devastating weapon, it can only fire one shot before needing to reload. As it is also unique, you can only reload it from alien Ammo Packs (or Supply Drops). If you're trying to hold onto it during a long segment, keep in mind that dropping an empty weapon causes it to de-spawn.

It's true power lies in it's Charge Shot: When aiming, hold down the fire button to charge it's energy. As you do so, it will go from a stage 1 shot, that can be fired right away, all the way to a stage 3/Max shot! This devastating attack destroys any Outsider alien, and can halve a Sectopod's HP in one screen-bleaching blast.

Without the clip-expanding pack, it only carries 3 shots total, so make them count!