A species of parasite was found in the Mariana Trench after it escaped the destruction of its homeworld.
This parasite was aquatic and could transfer from host to host in the water. They could sense the memories of potential hosts from afar. They flourished in water rich in hydrogen sulphide and would die if removed from it. They could infect many 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 depths and gain enough strength 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. A fifty year old host would last the parasite 50 years. (AUDIO: Submission)
About 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 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.
In 1959, the Guernica came to explore the Mariana Trench. The parasite tried to infect the crew members, sneaking in via 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 his memories were used up in 2009, the parasite called for help and was heard across the world. When the Torchwood Three team came to investigate, it crashed their submarine and tried to infect them. 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, and it lifted away from the black smoker. The parasite suffocated and Carlie was free. (AUDIO: Submission)