Parasite (Submission)

An unknown species of parasite was found in the Mariana Trench after it escaped the destruction of its homeworld.

Biology
The parasite was aquatic and could transfer from host to host if they were in the water. They survived in water rich in hydrogen sulphide and would die if removed from it. They could infect different species, including humans, but some specific people were unsuitable (for example, those taking amitriptyline).

Once infected, the host gained a variety of abilities. They could survive underwater at great pressure and gain enough strength to to dent steel with their bare hands. They could make noises with enough force to travel the whole world and to damage electronics. In return, the parasite fed on the host's memories. This was a 1:1 ratio, meaning a fifty year old host would last the parasite 50 years. They could also sense the memories of potential hosts from a distance. (BBCR: Submission)

History
Roughly 150 million BC, the parasite's homeworld was being destroyed by its dying star. Two of the last survivors planned to escape, but one refused to wait for the other and escaped alone. It crashed on Earth in the Mariana Trench and was stuck in the radius of a black smoker volcano, which produced the hydrogen sulphide it needed to survive.

Millions of years later, in 1959, the Guernica came to explore the Mariana Trench. The parasite tried to infect the crew members by sneaking in a leak in the ballast, but Henry Goddard's usage of amitriptyline made him unsuitable. Instead, it took Sam Doyle and fed on his memories.

After the memories were used up in 2009, the parasite made a call for help that was heard across the world. When the Torchwood Three team came to investigate, it crashed their submarine and planned to infect them next. When they tried to escape in the Guernica, the parasite got in through the same leak and infected Carlie Roberts. The Guernica had been set to rise, meaning it lifted away from the black smoker, causing the parasite to suffocate to death and free Carlie. (BBCR: Submission)