The Nestene Consciousness
The Nestene Consciousness has no physical form and exists as a form of intelligence that colonises other planets. It often take on a physical form when trying to colonise a planet, and prefers a tentacled form such as an octopus.
The Nestene Consciousness first arrived on Earth split amongst 50 plastic meteorites that arrived through a super heated corridor. Once on Earth they were collected and brought together to form the complete consciousness. The Nestene Consciousness later arrived on Earth around the year 2005, but the method of arrival is undetermined.
The Nestene Consciousness has an affinity for plastic, and is able to exert control over plastic. The Nestene Consciousness, in the first two attacks on Earth, used a plastics factory to create plastic mannequins - Autons. The first Nestene Consciousness colonisation attempt was thwarted by the Third Doctor when he was able to jam the Nestene Consciousness's control over plastic and short circuit the consciousness that was on Earth.
The Master tried to help the Nestene Consciousness return to Earth by using a radio telescope. The Doctor managed to convince the Master that once on Earth the Nestene Consciousness would destroy everyone, including the Master. Together the two Time Lords used the radio telescope to banish the Nestene Consciousness to outer space.
Later the ninth Doctor, after failing to reason with the Nestene Consciousness, was able to destroy the Nestene Consciousness by using a liquid, dubbed anti-plastic by Rose.