Light (Ghost Light)

Light was a traveller who journeyed through the universe making a Catalogue of Life, described by the Seventh Doctor as an "evil older than Time itself."

Characterisitics
Light's form depended upon his environment. When he awoke he "naturalised" in an angelic human form, majestic and imposing, glowing with a yellow light and wearing a yellow robe.

In 1883, the Seventh Doctor told Ace that he moved at the speed of thought, teleporting in a flash of lightning and thunder. Additionally, he possessed telekinesis and could telepathically kill someone, causing them to glow blue with electricity. He could also highlight microscopic organisms and dismember, petrify, and convert humans into primordial soup.

His angelic appearance wildly contrasted with his bewildered, frustrated behaviour, doddering speech pattern and screeching timbre. Depending on his mood, he could speak in a timid voice or an angered voice. Light had an absolute hatred of change, even finding the idea of himself changing to be nonsense. (TV: Ghost Light)

Biography
Light was an Eternal (PROSE: Divided Loyalties) who travelled the universe, taking it upon himself to catalogue all organic life. Assisting him in his Catalogue of Life was the Survey kept under Control, and all three travelled in Light's ship.

On prehistoric Earth, he faithfully catalogued all life, including taking the Neanderthal Nimrod as a specimen, having been worshipped by Neanderthals as "the Burning One." But no sooner had he finished cataloging life on Earth than it all started changing. For unknown reasons Light went into hibernation and the ship remained in what would become Africa. Meanwhile, the Survey got out of control and began adopting different forms of dominant life, but remained fearful of Lights' return.

While hunting in Africa, Redvers Fenn-Cooper claimed to have seen a dazzling light, noting that it had wings and other features. Afterwards, the ship was moved below Gabriel Chase in Perivale. Survey became "Josiah Samuel Smith" and ran the house, keeping Control locked in the ship.

After the Seventh Doctor and Ace arrived, Ace accidentally awoke Light and the Doctor "promised" Control her "freeness". Asleep for millennia, Light didn't believe he was on Earth because it was different. To determine this, he killed a maid and took apart to see how she worked. He became angry that even microbes were evolving; everything on Earth had changed and his Catalogue was ruined. He then killed Inspector Mackenzie, turning him into primordial soup, which he planned to do to Earth.

After turning Lady Pritchard and her daughter Gwendoline to stone, he decided Earth's fate. Since the primordial soup would start to evolve again, he would stop the change, eradicating all organic life in the "firestorm program." The Seventh Doctor showed Light that he himself was evolving, since he was no longer his original shape. He asked Light why his Catalogue didn't include gryphons, basilisks, dragons, bandersnatches, slithy toves, the crowned Saxe-Coburg and other creatures Light didn't know. Confused as to why these weren't in his Catalogue, Light refused to accept that even he had evolved. To prevent himself from changing, Light dispersed himself. (TV: Ghost Light)

Behind the scenes
The Doctor's comment that Light travels at the speed of thought is a play on the fact that thought supposedly travels at the speed of light.