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 more powerful than any other monster in the game, including the Cyberdemon. She is visually the closest monster in Doom 64 to the Arch-Vile. 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.

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.