- You may be looking for the Earth spacecraft visited by the Sixth Doctor or the transcendental beings of the same name.
The Leviathan was a Dæmon spacecraft which ran on nuclear power. It was launched from Dæmos on a mission under Commander Azal to learn if the universe had an edge. In the third month of the fourth year of the mission, the Leviathan encountered an outside force which reduced its speed until it came to stop in a misty white void. The crew went outside to explore, but when Azal went further ahead and vanished, they decided to leave. (COMIC: Voyage to the Edge of the Universe)