- You may be looking for Dax Fex Fize Slitheen.
The Dax were an underwater species from the destroyed planet Daxus.
At an unknown future date, some Dax were living in an abyss near the Corinthian Project. They remained hidden, until discovered by the Tenth Doctor in a submarine. The project was about to blow up, but the Dax helped to save it. (NOTVALID: The Corinthian Project)