Light was an extremely powerful, mentally unstable, almost God-like alien being in the Doctor Who episode "Ghost Light".
Long ago, he took a survey of all organic life in the universe, but almost as soon as he finished 'it all started changing.' Light went into hibernation in his stone spaceship, hidden in the basement of Gabriel Chase in Perivale Village, London, while members of his cargo took over the house in 1881 and attempted to integrate into Victorian high society.
After being awoken in 1883, following the arrival of the investigating Seventh Doctor and his companion Ace, Light took the form of an "angel" and began to campaign against evolution and change, deciding to destroy all life so that his catalogue would never be out of date again. He managed to kill three people: Gwendoline Pritchard, Margaret Pritchard and Inspector Mackenzie. Before he could carry out his plan though, the Doctor told Light that it was futile to oppose evolution and that even he was changing. Light reacted by turning his power on himself to stop himself changing, thus Light 'dissipated' in the main hallway of the house.
Paradoxically, the emotionally volatile Ace, then 13 years old, burnt down Gabriel Chase in 1983 after sensing an evil presence. This was confirmed by the Doctor to the older Ace to be an "echo" of Light following his dispersal 100 years earlier.
Light's majestic appearance sharply contrasts with his doddering, confused behaviour. The Doctor demonstrates near perfect understanding of Light pointing to where Light will appear and using his mental instability against him.