Great Intelligence

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

History
Yog-Sothoth was a being that existed before the present universe and 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. He along with his kind shunted themselves into a parallel dimension in order to pass on into the current Universe where he 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 would work and thus waged a campaign on a million inhabited worlds. Yog-Sothoth was under a considerable amount of pressure from the other Great Old Ones who themselves embarked on similar campaigns and conquered other planets. (MA: Millennial Rites)

It had been exiled from another dimension, and was forced to wander the universe in an attempt 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 on astral plane. In 1935, the Second Doctor, a friend of the lama's, as well as 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, ensnaring the Doctor's TARDIS in space and forcing it to land in the London Underground, where, 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, attempting to use a conversion headset to take over and possess 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. (MA: 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, and prepared a counterspell designed to force it back into its own reality. However, this instead had destructive effects, including merging the Intelligence and the benevolent god Saraquazel into a single malevolent being, and altering reality around London to a realm partly obeying the laws of the Intelligence's universe. This was undone, and the Intelligence was thrust back to its prison. (MA: Millennial Rites)

Powers
The Great Intelligence had no physical existence, which meant it was incapable of interacting with the rest of the universe. It could possess living creatures, such as humans and also used control spheres to form a locus for the Intelligence. Also, it manifested as webbing in order to trap the Doctor's TARDIS. (DW: The Web of Fear)

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