Vagary

The Vagary is an enemy unique to Doom 3, presumably being inspired by the Spider Mastermind from the classic games as a spider-themed, psychic demon boss.

She is a large spider-like demon with an upper half of a woman's torso, and a transparent abdomen that contains a malformed Mancubus fetus gestating inside. She emits sharp throaty screeches similar to a distorted female voice when spotting the player.

This monster is first encountered at the end of Alpha Labs Sector 4 as a mini-boss, and re-appears in the near end game of Caverns Area 2, accompanied by a large swarm of Trites.

Combat characteristics
The Vagary primarily attacks through telekinesis. She will levitate any objects to telepathically throw them at the player. Occasionally, there are barbed stones around the vicinity which she will use to deliver more damage, although she will resort to levitating any other form of debris as well if given the opportunity.

At close range, she will resort to melee, by using her front legs to slash at the player.

Tactical analysis
When the player first meets a Vagary, he/she should have picked up the chaingun not far behind. A prolonged chaingun salvo should send the Vagary packing in under a minute. It's also possible to use the plasma gun if it was picked up earlier in the level, as this will quickly deplete her health.

Killing the Vagary should not give the player any trouble as long as he/she keeps on dodging the projectiles the Vagary throws by telekinesis. It is possible to easily dodge her projectiles by simply listening to the sound cue(s) and crouching at the right moment.

A pair of Vagaries will attack the player at the very end of the game, just prior to the final battle with the Cyberdemon. By this point, the player should have the Soul Cube, and should be prepared to use it. The best course of action is to use the Soul Cube and then either the chaingun, plasma gun, or rocket launcher to finish her off. A BFG shot might be used for players seeking a quick end. If possible, the first Vagary should be killed using the Soul Cube and weapons fire before the second Vagary makes her appearance several seconds after the first one.

This time, she is accompanied by Trites, so the player should have a shotgun or machine gun ready to eliminate the swarm before tackling their mother. Note that the Trites are a particular nuisance when using the Soul Cube, sometimes being targeted instead of the Vagary, and the BFG, as they sometimes destroy the chip it fires before detonation, which can hurt the player if he/she is too close to the explosion.

The Lost Mission
In the Lost Mission campaign of the Doom 3 BFG Edition, a Vagary appears as a mid-level miniboss in Hell. This time, though, she cannot use her telekinetic attack, due to the absence of loose objects to throw, and is instead limited to running at the player and using her melee attack. She is also accompanied by several Cherubs which will attack in tandem with her.

Trivia

 * In the Making of Doom 3 book, the equation "Sexy + Gross = Creepy" was used to describe the driving concept behind the Vagary. She is also the only monster in Doom 3 that is clearly female, while the others are either male or genderless.
 * Despite being labelled as the first official boss in Doom 3, the Vagary is technically a regular monster: she burns up when she dies, is vulnerable to headshots, and no cutscene is activated upon her death. However, she serves as a boss early in the game before the player collects more powerful weapons. The three Mancubi in Recycling Sector 2 and the pair of Hell Knights in Delta Labs 4, both play similar roles to a "boss fight" to introduce a more powerful regular monster.
 * The 2016 version of the Spider Mastermind has a telekinetic attack similar to the Vagary's own, which may have been inspired by it.
 * Fans often conjecture that it's the Vagary who speaks the phrase "They took my baby..." to the marine, among several other phrases, by using her telepathic power in order to mock or frighten him. The whispering voice is frequently heard when the player gets close to her. Ironically, she later appears in The Lost Mission with cherubs, which are baby-like demons.
 * The Vagary resembles a Vore from Quake and the Black Widow's second form in the Quake II mission pack Ground Zero, since both resemble humanoid spiders. Her design is also likely influenced by Driders, a dark elf/spider hybrid in the Dungeons & Dragons universe.