Aliens came to the Nemonites homeworld and terraformed it, driving them underground. Afterwards, the aliens killed them all, except for one. It took over one of the aliens and escaped in one of their spacecraft, which was able to travel through the Time Vortex.
The Tenth Doctor found the ship and pursued it to 1940 Earth. The Nemonite thought it was an ideal new home, and planned to take over the world. The Doctor offered to take it back home or to another planet, but the Nemonite refused.
Adult Nemonites were black and resembled large Earth leeches. They would attach to an organism and control that host completely. They would inject their spawn into the host, which would be vomited out later through the mouth in a viscous fluid containing thousands of the spawn. (AUDIO: The Nemonite Invasion)