Animus

The Animus, also known as Lloigor, Pwodarauk, the Voice, and the Intelligence, was a Great Old One from the pre-universe race known as the Lloigor. It was the only Lloigor to cross into this universe, afterwards landing on the planet Vortis. It could take over any living creature that was in contact with gold and manifested itself within an organic, self-healing palace called the Carsenome.

The First Doctor described the Animus as a creature of "almost unimaginable power" and "capable of great evil". (COMIC: Unnatural Selection)

Biology
The Animus resembled an octopus crossed with a spider or plant. It was able to create webbing to ensnare beings and generated a fungus-like substance which grew into its fortress known as the Carsenome. While the Animus could apparently communicate with the Zarbi, it spoke to other individuals through a helmet "hair-dryer" like device. In the centre of the Carsenome, the Animus was able to communicate with individuals directly. The Animus spoke with a feminine voice. (TV: The Web Planet)

The Animus consumed organisms through direct mental absorption, unless they were unfit and only suitable for gross organic matter supply. The tentacles of the Animus spread out and divided like roots, generating webbing and absorbing matter wherever they touched. If they possessed a strong enough will, individuals who had been absorbed could possess a tentacle and separate itself from the Animus. Additionally, fission could also occur if the Animus was ever divided, resulting in two Animuses. Before dying, the Animus could create a seed of itself as a precaution that would receive the memory of the destroyed Animus and grow into a new creature. It would speak of its predecessor as if it were a new individual altogether. (PROSE: Twilight of the Gods) The Animus that the First Doctor met beneath Victorian London was able to regrow to its former self from a single tendril. It considered itself the same Animus that was nearly destroyed by the Isop-tope. (COMIC: Unnatural Selection)

Powers
The Animus was able to draw off power from the planet Vortis by having its Carsenome as the north magnetic pole. This attracted several other planets or moons towards Vortis due to the Animus' use of isocryte. It was also able to siphon the water, drying up streams and leaving the planet barren with a thin, rarified atmosphere. It could control any creature that was in contact with gold and seemed able to telepathically communicate with its Zarbi slaves. The Animus could physically generate a light that seemed to either hypnotise and/or weaken individuals. (TV: The Web Planet)

It absorbed isocryte and could master its anti-gravitic properties to create force fields, gravity beams, and draw other celestial bodies to it, eventually mastering flight. It could also mentally control people directly, without gold, but this used a lot of energy. It could also create web beings that had four arms and were incredibly resilient. It also created a plant that could recreate web beings in the form of a dead organism as a copy that would be passable upon quick inspection. (PROSE: Twilight of the Gods)

Origins
According to several accounts, the Animus originated as an extradimensional being called Lloigor. (PROSE: All-Consuming Fire, Millennial Rites, The Dark Path) Accounts differed on whether the Animus was an Old One (PROSE: Twilight of the Gods) whose personal name was Lloigor (PROSE: Millennial Rites) or if Lloigor were a wider species with other members, with the Animus being one who broke through into the Doctor's universe. (PROSE: The Dark Path)

At some point, it landed on Vortis and enslaved the peaceful Zarbi, receiving its name from the Menoptera. The Animus was introduced to Vortis by Twel in an effort to interfere with Bris's and Ilex's experiment. (PROSE: Twilight of the Gods)

Vortis and the inital destruction
Later, the First Doctor, Ian, Barbara, and Vicki arrived and helped the Menoptera to attack the Animus. It seems its intention was to grow its Carsenome across the entire surface of Vortis, thereby taking over. However, upon encountering the Doctor, it became interested in reaching beyond the Isop Galaxy to "pluck from Earth its myriad techniques and take from Man his mastery of space." Barbara was able to use the Menoptera's secret weapon, the Isop-tope, to rapidly decay the Animus' cells. (TV: The Web Planet) The Isop-tope weapon shrivelled the Animus to "the slightest tendril". This was enough for the Animus to regrow. (COMIC: Unnatural Selection) After that, the travellers left the natives of Vortis to rebuild their lives peacefully. (TV: The Web Planet)

However, according to another account, the First Doctor was only with Ian and Barbara when they encountered the Animus. In this account, the Animus was the Zarbi's "brain centre", as opposed to it merely controlling the species. It would be destroyed by Barbara taking out one of its isotopes. (COMIC: The Zarbi Are Destroyed)

A sliver of the Animus survived in Barbara's mind, giving her psychic powers when she returned to 1965. These powers only lasted for a few months, until the Animus revealed itself and burnt out its power to destroy the set of the Roger and Elizabeth Adventures in Time and Space. (AUDIO: London, 1965)

Survival
The Animus, having regrown its Carsenome, was transported to the London Underground in 1868 with an army of Zarbi by a cloaked force so it could conquer Earth. As the army came through, the First Doctor and Thomas Huxley distracted it, as Ian drove a train into the Animus' Carsenome, killing it. (COMIC: Unnatural Selection)

Sometime after, the Second Doctor, Jamie McCrimmon, and Victoria Waterfield returned to Vortis and encountered the seed of the Animus. This Animus had a voice that was neither male nor female and viewed its predecessor's plan as full of mistakes. It manipulated the more radical leaders of both sides of the Rhumon in their civil war and had them mine isocryte, bringing the mineral to an island where the Animus awaited. Flooding an acid pool with the ocean, the Animus absorbed the isocryte and began spreading, planning to consume half of the world in a day. In an effort to damage it, a bomb actually divided the Animus into two beings. Oryl of the Gods of Light fought the Animus before being damaged and retreating.

Utilising holographic technology that would keep them from feeling the Animus but allowing them to physically attack it, the Doctor created giant versions of Jamie, the two Rhumon leaders, and Kretus the Menoptera. They attacked both Animus individuals and ripped them apart, crushing their heart/brains and killing the spawn of the Animus. (PROSE: Twilight of the Gods)

The Seventh Doctor, Lysandra Aristedes and Sally Morgan encountered the Animus during their travels in the black TARDIS. (AUDIO: Black and White)

During the Last Great Time War, a temporal conflict fought between the Dalek Empire and the Time Lords for the sake of the universe, (TV: Gridlock) the "Greater Animus" and its Carsenome were destroyed. (PROSE: Meet the Doctor, A Brief History of Time Lords)

Behind the scenes

 * Lloigor (and its twin Zhar) was created by H. P. Lovecraft's contemporary, August Derleth, who also made Hastur and the legendary Windigo into Great Old Ones. The singular reference in Doctor Who to a plural Lloigor comes from a later Colin Wilson story which suggests that the fictional Lloigor got its name from a very real group of beings.
 * The Dalek Handbook identifies the Greater Animus as one of the Higher Species due to their involvement with the Time War.