The Vagary is the first boss of Doom 3 and first encountered at the end of Alpha Labs Sector 4. Analogous to Classic Doom's Spider Mastermind, she has the lower half of a giant spider and the upper half of a woman. However, unlike the Spider Mastermind, the Vagary is weak compared to the other bosses in Doom 3 and returns in a later level (Caverns Area 2). Her attacks are a simple melee attack, as well as telekinesis, which she uses to lift and throw various heavy debris laying around at the player. Her transparent abdomen contains a malformed Mancubus fetus gestating inside.
The Vagary also resembles Quake's Vore and the Black Widow's second form from Quake II mission pack Ground Zero since both resemble humanoid spiders; both are likely influenced by Driders, dark elf/spider hybrids in the Dungeons & Dragons universe.
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 is 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.
In the Lost Mission campaign of the Doom 3 BFG Edition, a Vagary appears as a mid-level mini-boss 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.
- 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.
- Although her torso has breasts, these lack nipples.
- Despite being the first official boss in Doom 3, the Vagary is actually 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 play a somewhat similar role as a "boss fight" to introduce a more powerful regular monster.
- Because of this, the Guardian of Hell may technically be the first boss in Doom 3.
- Like some real spiders, the Vagary has six eyes. The two largest and most obvious are positioned roughly where human eyes would be. Two much smaller eyes are between these and slightly higher, straddling the top of the bridge of the nose, and another two of intermediate size are above those on the creature's forehead. All eyes are depicted without pupils and have black scleras (sclera is the solid "white" of the eye) mimicking the look of real-life spiders' eyes.
- In Resurrection of Evil, the Vagary is absent, although Trites and Ticks are present.
- Fans often conjecture that it is 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.