Mother demon

The Mother Demon is the final boss in Doom 64. She appears only on the last (non-secret) level, The Absolution. As her name implies, she gave birth to all the demons.

She is the most powerful monster in the game, surpassing the Cyberdemon. Her attack consists of powerful flame trails, followed by a set of 4 homing fireballs which split up when when they hit something.

Her body resembles that of a moth or like insect, yet she has no visible wings. She floats around with what appear to be vestigial legs, four arms that end with bright red hands and a head with a mouth full of teeth. Her eyes are red and small horns go down her back. Her only attack is casting magic spells at her enemy and she makes a ghastly snarl when moving around and a frightful shriek like a Banshee when she is attacked. She explodes when she dies.

In the backstory, the Mother Demon appeared in an old relay satellite, battered by neutrons and quarantined by massive levels of radiation. When her presence was made, the satellite activated and sent a message to Earth. Her presence came with many demons and though their energy signatures were caught, the Mother Demon used the radiation levels to escape detection. She has rejuvenation powers and whenever she walked by a corpse of a demon, it was systematically brought back to life. It also made them stronger than before.

After she is killed, the Marine decides to remain in Hell to ensure no demon ever rises again.

Tactical analysis
The Mother Demon will kill you instantly if you come in battle unprepared. Her flame trail, when hit, launches you into the air, making it harder for you to control your movements. At the same time, she will launch 4 homing rockets at you, being the only monster to perform 2 attacks at the same time. Even though 1 Homing Missile is not as strong as 1 Rocket, getting hit by 1 of them will bounce you back pretty far and getting hit by all 4 of them at once can be fatal. Dodging the rockets can be a hassle (though you can still hide on the walls like how you fight the Revenant), especially when hit by the flame trail or the first rocket.

The most effective way to defeat the Mother Demon is to use the fully-equipped Unmaker. Not only will the Unmaker do a lot of damage at rapid-firing speeds, but will also stun her to prevent her from using her 4 homing missiles. She will still attack with the flame trails as the only attack to avoid.

However, if you did not receive all 3 of the Demon Keys needed to fully equip the Unmaker, the battle against the Mother Demon is very difficult. To make matters worse, varying upon how many keys you collected if you did not collect all 3, your previous battle before facing her may have consumed a lot of ammunition. Your only real choice is to use the BFG 9000, but it requires precise timing and a lot of Plasma Cells conserved from the previous battle against the horde of enemies. The Rocket Launcher is the next best alternative, but defeating the Mother Demon with it takes longer and is more difficult.

Other games
There is a slight appearance in Doom 3. When your open your PDA, after you obtain an audio log in the Dig Site, there is mention of a demon that gave birth to all other demons, though you never see it.

Also in Resurrection of Evil, when you bring the stone artifact to the scientist for scanning, during his conversation with Dr. McNeil, a stone tablet bearing a demon that looks like the Mother Demon can be seen.