Vagary

Vagary is the first boss of Doom 3. She has the legs of a giant spider and the torso 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. Her attacks are simple mêlée and telekinesis. Her abdomen contains a malformed Mancubus fetus. Later on in the game, two more Vagaries are fought.

The Vagary also resembles Quake's Vore enemy since both resemble humanoid spiders; both are likely influenced by Driders, 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.

The second time the player meets Vagary, they 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.

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.