The Elysium parasites, as Ace called them, were a species of cosmic parasites that, were they not restricted by their metabolism, could have overrun the universe. They referred to themselves as "Life," distinguishing between "Life" and "life".
The Artifact located in the Elysium system was a member of the species. It contained an entire ecosystem within itself, and extended into more than eleven dimensions, even containing portals akin to singularities. Their eggs were the size of planets and were laid in gas giants, taking hundreds of years to mature and hatch. When he became psychically linked to the Artifact, the Seventh Doctor was able to make it so that all future offspring would be symbiotes rather than parasites. (PROSE: Parasite)