Vagary

The vagary is the first boss of Doom 3 and is 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 spiderdemon in classic Doom. However, unlike the spiderdemon, 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 pieces of heavy debris lying around the level at the player. Her transparent abdomen contains a malformed mancubus fetus gestating inside.

A vagary makes an appearance as a mini-boss in Hell during the Lost Mission from Doom 3: BFG Edition.

The vagary also resembles the vore enemy from Quake- both resemble humanoid spiders. Both are likely influenced by Driders, dark elf/spider hybrids in the  universe.

Tactical analysis
When the player first meets a vagary, he or she 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 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 route of attack 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.
 * Unlike all other bosses from Doom 3, she burns up when she dies and no cut scene 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 head shots.
 * Although her torso has breasts, these lack nipples.
 * The vagary's upper half has four eyes, being two regular "human" eyes, with other two smaller, spider-like eyes above them, placed on her forehead, at the eyelash level. All the four eyes are pupil-less with black sclera.
 * In Resurrection of Evil, the vagary does not appear, but the trite does.
 * Fans often conjecture that it is the vagary who speaks the phrase, "They took my baby," to the marine by using its telepathic power in order to mock or scare him. The whispering voice is frequently heard when the player gets close to her.