Rutan scout

A Rutan scout crash landed on Earth in the 1900s. It was sent to establish a base with a power source and to create a beacon to bring the invasion fleet to Earth.

Having studied all planets in the Sol system, Earth was chosen as the best to serve the Rutan Empire's plot to destroy the Sontarans. The scout made its way to Fang Rock and infiltrated its lighthouse.

There it established the beacon and began to kill the lighthouse keepers and shipwreck survivors, one by one. Using recent Rutan technological developments, the Scout shape-shifted into the form of the lighthouse keeper Reuben. It electrocuted the others.

After a short talk with the Fourth Doctor, the Rutan attempted to take the lighthouse. Leela destroyed it with a makeshift mortar and gloated over the scout as it lay dying. (TV: Horror of Fang Rock)

Three regenerations later, the Doctor was forgiven by the Rutan Host for killing the scout. (PROSE: Shakedown)