Bruydac

The Bruydac were a species from the planet of the same name. They were roughly humanoid, but quite ugly to human sensibilities. They had a history of invading other planets, using devices implanted in brainstems of the natives to possess their bodies and thus infiltrate the planet's society. The Bruydac's own body would remain unconscious within its ship.

A controlling Bruydac had full access to its host's memories and was influenced by their personality to some extent, considering itself to be that person on some level, but retained Bruydac motivations and would calmly sacrifice its current body once it served its purpose. A Bruydac with implants in multiple people could jump between them at will. While in a host body, it needed to cannibalize the brains of other people. Starfish-like creatures grew inside the hosts' stomachs, which could be vomited forth as living weapons.

A Bruydac ship came partly through the Cardiff rift to Earth in 2007, causing massive rainfall. The Bruydac warrior inside possessed several people and committed murders in the city of Cardiff by devouring brains, while gathering components to launch its ship all the way through the rift. It was stopped by Torchwood 3, who killed its Bruydac body and trapped it in the body of Jack Harkness, who was then drowned. Jack revived, the Bruydac did not. (TWN: Another Life)