Light was a traveller of the universe. He took it upon himself to catalogue all organic matter. He took Nimrod, a member of the Neanderthal race, as a specimen for his catalogue. Redvers Fenn-Cooper claimed to have seen a dazzling light while hunting in Africa, noting that it had wings and other features similar to those of Light. Light had slept for centuries in his spaceship and life had evolved.
Light was angry that his catalogue was ruined because life evolved. He decided to destroy all organic matter so it would stop changing and his catalogue would be complete.
Light killed a maid and took her apart in order to see how her body worked. He killed Inspector Mackenzie, turning him into soup and turned Lady Pritchard and her daughter Gwendoline to stone. The Seventh Doctor showed Light that he himself was evolving. Light, in an effort to prevent himself from changing, dispersed himself. (TV: Ghost Light)
Light's appearance was majestic and imposing, which wildly contrasted with his bewildered, frustrated behaviour, doddering speech pattern and screeching timbre. Light's form depended on his environment. When he awoke he appeared to be a human who glowed with a yellow light and wore a yellow robe. The Seventh Doctor told Ace that he moved at the speed of thought. (TV: Ghost Light)