Vagary

The Vagary is the first boss of Doom 3 and first encountered at the end of Alpha Labs Sector 4. She has the lower half of a giant spider and the upper half of a woman. She is analogous to the Spider Mastermind in Classic Doom. 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 simple melee, as well as telekinesis which she uses to lift and throw various heavy debris laying around the level at the player. Its transparent abdomen contains a malformed Mancubus fetus gestating inside. She/it maybe was the voice who said "They took my baby" to the Marine by using its mind power in order to mock or scare him, the whispering voice frequently being heard when the player gets close to her.

The Vagary also resembles Quake's Vore enemy since both resemble humanoid spiders; both are likely influenced by Driders, dark elf/spider hybrids in the Dungeons & Dragons universe.

Tactical analysis
When the player first meets Vagary, they should have picked up the chaingun not far behind. A prolonged chaingun salvo should send the spider packing in under a minute. It is also possible to use the plasma gun if it was picked it up earlier in the level, as this will quickly deplete her health. Killing the Vagary as quickly as possible is very important, as the objects she throws using telekinesis are very difficult to dodge, although they can be avoided somewhat successfully by sprint-running sideways.

A pair of Vagarys 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. Best route of option 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 its appearance several seconds after the first one.

This time, she is accompanied by Trites, so the player should have a shotgun or machine gun handy ready to eliminate the swarm before tackling their mother (note: the Trites are a particular nuisance when using the Soul Cube, sometimes being targeted instead of Vagary, and the BFG, as they sometimes destroy the chip it fires before detonation).

Trivia
In the Making of Doom 3 book, the equation "Sexy + Gross = Creepy" was used to describe the driving concept behind the Vagary.

Also, unlike all other bosses from Doom 3, she burns up when she dies and no cutscene is activated on her death. She is also the only boss monster that appears multiple times. Due to these two features, she could be called a "miniboss". She is also the only female boss in Doom 3, unlike other bosses who are either male or genderless.

The Vagary is the only boss that is vulnerable to headshots.