Unmaker

Unmaker 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 version
In Doom 64 it is the strongest weapon when equipped with all three Demon Keys, as well as the only new weapon in the game. It is especially rare because it can only be found in a few levels, the first one being Level 29, Outpost Omega (a secret level). The first regular level where it can be found is Level 12 (Altar of Pain). It shoots a powerful red laser, and it can be upgraded when the player finds the Demon Keys: Like the BFG and the Plasma rifle, it uses cells as ammo.
 * The first one increases the laser speed.
 * The second key adds a second laser
 * The third one makes the weapon fire three simultaneous lasers.

When collected, the message says: "What the !@#%* is this!"

A fully equiped Unmaker can critically increase the chance of pain rate, killing the Cyberdemon even before he gets a chance to shoot his rockets due to pain stun.