Great Intelligence

The Great Intelligence (also known as Yog-Sothoth) was a disembodied sentience who attempted to find a body and physical existence.

Existence
In the universe before the present one, Yog-Sothoth had a body that was a mass of tentacles and mouths. (MA: Millennial Rites) Like the other Old Ones, it may have lost its body when it ran from the Fendahl. (NA: White Darkness)

Afterwards, the Intelligence constantly sought physical form, but it did maintain a somewhat basic or simple manifestation. When not using a living being, it existed as a three sided pyramid composed of control spheres. (DW: The Abominable Snowmen) When forcebly summoned to Earth by Anne Travers, and being combined with three sets of physical laws, the Great Intelligence was an emerald tetrahedron and, because of Travers' meddling, was merged with the god Saraquazel. (MA: Millennial Rites)

Powers
The Great Intelligence had no physical existence and thus relied on possessing living creatures in order to manipulate its environment. It existed on the astral plane and could enter the people it encountered. Taking control of Padmasambhava, it allowed him to live over 300 years while it used him to create Robot Yeti and can also reanimated a dead body like Staff Sgt. Arnold. A portion of the Intelligence was held within each control sphere and appeared to act independently, somewhat. (DW: The Abominable Snowmen) The Intelligence could also manifest as a poisonous webbing, using it to take over parts of London as well as to trap the Doctor's TARDIS, (DW: The Web of Fear) and could possess the New World University computer network, although this kept it bound to the network. (MA: Downtime)

History
Yog-Sothoth was a being who existed before the present universe. It was a member of a race of beings called the Great Old Ones, who were the equivalent of the Time Lords in their native realm. They shunted themselves into a parallel dimension to pass into the current Universe. Yog-Sothoth discovered he had gained god-like powers. Being the military strategist of his people. the Great Intelligence decided to see if various stratagems he had considered and played on computers would work, and waged a campaign on a million inhabited worlds. He used the Hisk version of Koala bears on Hiskith and domestic animals equivalent to dogs on Danos. Though the Yeti had failed to take over Earth, he was under pressure from the other Great Old Ones ,who themselves embarked on similar campaigns and conquered other planets, forcing him to use the Yeti in London, an environment they were not suited to. (MA: Millennial Rites)

Yog-Sothoth had been exiled from another dimension, and was forced to wander the universe to find a body to possess. (DWN: Doctor Who and the Web of Fear)

At some point, the Great Intelligence possessed the Tibetan lama Padmasambhava while he was travelling the astral plane. In 1935, the Second Doctor, a friend of the lama's, with Edward Travers, a westerner determined to find the Yeti, intervened. Padmasambhava's physical body died when the Intelligence melted away. (DW: The Abominable Snowmen)

Thirty-five years or so later, the robot Yeti re-activated and the Intelligence manifested as webbing. It ensnared the Doctor's TARDIS in space and forced it to land in the London Underground. Reunited with Travers, the Doctor assisted British military in their battles with the Yeti. The Intelligence re-animated and possessed the corpse of Staff Sergeant Arnold, using him to track the Doctor's actions. The Intelligence captured the Doctor and tried to use a conversion headset to take over the Doctor's body. The Doctor attempted to reverse the process, allowing him to absorb the Intelligence and destroy it. When the control spheres that formed the focus of the Intelligence were smashed by Jamie McCrimmon, the Intelligence vanished, powerless but still alive. (DW: The Web of Fear)

The Intelligence later contacted the Doctor's former companion Victoria Waterfield and manipulated her into using computers to return to physical existence. (RP: Downtime) Anne Travers, left traumatised by the Intelligence's first attempts to enter the current universe, was left believing millionaire Ashley Chapel would try to use a special program, the Millennium Codex, to summon the Intelligence to Earth. He prepared a counterspell to force it back into its own reality. However, this had destructive effects, including merging the Intelligence and the benevolent god Saraquazel into a single malevolent being. It altered reality around London to a realm partly obeying the laws of the Intelligence's universe. This was undone, and the Intelligence was thrust back in its prison. (MA: Millennial Rites)

The Seventh Doctor, Lysandra Aristedes and Sally Morgan encountered the Great Intelligence during their travels in the black TARDIS. (BFA: Protect and Survive)

Behind the scenes
Yog-Sothoth is the name of a fictional deity created by H. P. Lovecraft. The name first appears in The Case of Charles Dexter Ward. It was implied to be the most powerful being in the universe and described as existing outside of the space-time continuum.

Writer Neil Gaiman disclosed in Doctor Who Magazine that earlier drafts of his script for The Doctor's Wife implied that House, the villain of that story, was actually the Great Intelligence. These hints did not make it into the episode as aired.