Unmaker

Unmaker or the Alien Laser is a weapon from Doom Bible which was originally intended to be in Doom, but was subsequently removed. However, it later appeared in Doom 64.

According to the Doom Bible, the Unmaker was intended to be a "demon-tech weapon that hurts pure demons a lot, demon-humans very little, tech demons some. Made of demon bones. (...) The Dark Claw and Unmaker feed on human souls. Killing possessed humans or hellslaves allows the weapons to feed". This idea was later used in Doom 3's Soul Cube.

Doom 64
Doom 64 is the only game that this weapon can be found and officially used in. It also replaces the BFG9000 as the final weapon in the game. Before the weapons in Doom (2016), it is also the only weapon in the Doom series to receive upgrades (the Pistol to Chaingun and Shotgun to Super Shotgun do not count, as they take their own weapon slots, unlike the Unmaker's different power variations).

The Unmaker is extremely rare because it can only be found in a few levels, the first one being MAP29: Outpost Omega (a secret level). The first regular level where it can be found is MAP12: Altar Of Pain. It shoots a powerful red laser that pierces through enemies, and it can be upgraded when the player finds the Demon Keys: Like the BFG and the Plasma rifle, it uses cells as ammo.
 * Having no demon keys will cause the laser weapon to shoot moderately slow.
 * The first one increases the laser speed, making it a rapid-firing weapon.
 * The second key adds a second laser, slightly spreaded.
 * The third one makes the weapon fire three simultaneous lasers, greatly spreaded for coverage.

When collected, the message says: "What the Fuck is this!"

The damage varies a lot per shot based on the following:
 * The number of keys you have.
 * Simultaneous shots (2nd and 3rd keys) do not technically mean x2 and x3 on damage. The power is reduced when the 2nd or 3rd simultaneous laser is shot.
 * Varies upon enemy and luck. The laser shot may be powerful enough to kill a Demon in 1 shot. Sometimes, it may not kill a Zombieman in 1 shot.

The Unmaker, even without any of the demon keys, has some advantages over the Plasma Rifle:
 * More overall damage per shot, about 2x more damage per shot.
 * Thin shots, meaning, it can go through small gaps the Plasma Gun cannot. This can be a disadvantage however, as great accuracy will be required to successfully hit smaller monsters.
 * Smaller on-screen shots, meaning you will be less blinded, unlike the bigger blue ball shots from the Plasma Rifle.

The Unmaker with two or more demon keys has more advantages over the BFG9000:
 * More versatile. The BFG9000 is limited in combatting against a good amount of enemies or bosses; wasting 40 shots for smaller grunts is wasteful. The Unmaker can go either way, thanks to its spreadfire function.
 * The Damage per shot is often in the Unmaker's favor most of the time. While the BFG can potentially kill the Cyberdemon in 2 shots, the Unmaker, with 100% accuracy, can kill the Cyberdemon in about 81 shots.
 * Speed and risk. You have to be dead-on with the very slow BFG shots; otherwise, you can be a sitting duck. The Unmaker has no recoil openings.
 * Significantly less cell ammo is used.
 * The Unmaker's three lasers' capability to hit targets independentally, coupled with the high rate of fire, makes it faster to clear rooms compared to the BFG9000s' brief tracer attack.