Zalphon

The Zalphons were a green, gaseous-ectoplasmic species, which the Tenth Doctor also referred to as “mind skippers”. They were parasitic, travelling the universe to feed on brain energy and thought patterns. They could drain their victims of their brain energy before moving on to their next host, or occupy and control their host for extended periods of time. While being a powerful psychic species, the Zalphon were not able to read Time Lord thought patterns.

On 12 July 1979, a Zalphon scout ship crashed into Skylab, and two escape pods from it crash-landed on Earth. Upon seeing the escape pods, and realising that the shout ship had come from Earth, the Doctor realised that the Zalphons had already begun invading the planet. He tracked them to Sydney, where they had built a mothership in the form of the Sydney Opera House. The Doctor foiled their invasion attempt by directing the Zalphon ship to Antarctica, where the ectoplasmic species would freeze into ice crystals. (PROSE: Bennelong Point)