Unmaker

The Unmaker is a weapon in Doom 64 of demonic origin, inscribed with a pentagram and made up of parts of the spine and rib bones of a demon. It fires powerful red lasers by consuming cells as ammo. The official name of the weapon is currently unknown and unverifiable - the name "Unmaker" has long been applied to the weapon due to analogy with a similar concept in the Doom Bible.

The Doom Bible concept for the Unmaker was originally intended to be in the pre-planned commercial sequel to Doom itself. 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".

The idea of a weapon being charged by kills was later used in Doom 3's Soul Cube, and the concept of a Hell-wrought weapon fed by human souls also occurs in the form of the Artifact.

Doom 64 version
The Unmaker is the only new weapon in Doom 64. It begins as the second strongest weapon behind the BFG9000, with slightly better damage but a slower firing rate than the plasma gun. It is especially rare and 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. However, uniquely amongst Doom weaponry, the Unmaker can be upgraded by finding the three mysterious and well-protected Demon Key artifacts:
 * The first one increases the laser speed.
 * The second key adds a second laser
 * The third one makes the weapon fire three simultaneous lasers.

The fully equipped Unmaker critically increases the chance of pain-stun, dispatching the Cyberdemon and even the mighty Mother Demon with impunity. While the BFG can dole out higher damage and strike more targets at a single time, the Unmaker arguably surpasses it in utility in the late game when it is at its full strength, strong enemies come in singles or small groups, and cell ammo is at a premium.

Name
The weapon is never officially named within the game itself, and is kept a secret by the instruction manual. The Unmaker weapon sprite is named and the pick-up sprite is named. It is likewise referred to by several unofficial or tentative names in pre-release previews, walkthroughs, and reviews. These include the following: When the weapon is collected, the marine simply exclaims, "What the !@#%* is this!"
 * "laser rifle"
 * "alien laser" (a name also referenced by level designer Tim Heydelaar)
 * "superlaser"
 * "laser gun"
 * "demon laser"

Trivia

 * The reaper in Doom (2016) has a charged attack which fires a similar laser-like beam of Hell energy.
 * One pre-release screenshot shows what is presumably an early version of the Unmaker. The publication itself mentions there could be "possibly new weapons and monsters" in Doom 64. Though Tim Heydelaar believes this picture is of an early rocket launcher model, earlier screenshots which display the rocket launcher sprite as it appears in the final game would seem to contradict that notion.

https://www.doomworld.com/forum/post/1955898

Alpha version
As a weapon in the Doom alphas, the Unmaker appears only in Doom v0.2, and there only in the form of two unused lumps for the game's helmet visor HUD: contains the name of the weapon, and  contains a silhouette. After this, there are no further references to the weapon concept. It was meant by Tom Hall to occur as a new weapon in the commercial sequel to Doom and not in the first game itself, which is likely why its concept was quickly dropped.