A first in the Doom series, the ultimate weapon in Doom 64 isn't actually the BFG 900, but a new gun called the Unmaker. Originally planned for the first Doom in 1993, the gun was added as an Easter Egg/bonus by Midway for Doom's N64 debut. While you can acquire it during regular gameplay, you'll need to locate three Demon Artifacts to make it effective in combat.
Finding these keys requires you access the secret levels in Doom 64. We have a specific guide to help you there, but just getting to the levels isn't enough. You'll need to solve little puzzles to nab each key, ensuring you get the beastly Unmaker and can topple the final boss in seconds. If you're struggling to grab any specific key, this guide will help you.
Simply click the embedded videos to be taken to their respective timestamps.
Map29: Outpost Omega
The first Demon Artifact is found on the first secret level. Once you have both the yellow and red keys in your possession, you'll be able to acquire it. Go back to the room that was locked by the blue key and jump down the first drop. Turn left, then drop down there. You'll need to make some running jumps to a switch that was hidden by a wall (it probably dropped the first time you were there).
Hit that switch, then turn around. Run back across both drops and right an immediate right onto an elevator. It will soon bring you up and you'll need to shoot a switch to drop another platform. Repeat the same process -running across the pits, hitting the switch, jumping on the elevator, shooting the button- and the door for the Demon Artifact will now be open. You'll need the red key, but you should already have that.
If you're having trouble finding the red key, it is located in the room where you get surrounded by Mancubi. From the entrance, turn to your right and press the monitor on the wall. It won't make a noise, but a wall will drop that has the red key in it.
Map30: The Lair
The second Demon Artifact is located on the second secret level. It's probably the easiest to obtain of the three. At one point during this level, you'll spot it perched up high on a small platform. You'll need to kill all of the enemies before hitting a switch to reveal some stairs...and some more enemies. Once taking care of them, you'll proceed to shoot two switches overlooking the courtyard that will drop the key for you to grab. Follow the video if you're lost.
Map31: In The Void
The final Demon Artifact is found on the third secret level. It is the most involved of all of the keys to find. Play through the level as normal until you acquire the yellow key. Head for the main exit (which requires the yellow key) and walk through the arch a bit. The screen will shake and you'll now need to return to the blue key switch -the Demon Key was visible in this room-.
In front of the Demon Key should be a new switch. Once you throw this, you'll have to run back to where you picked up the blue key (this was behind the red key door). This requires you to enter some teleporters. If you're fast enough, you'll be able to nab the Demon Key at the end of a long pathway. If not, you'll need to repeat the process.