Elysium

Elysium was a name applied to possibly two places -- or one. One account refers to Elysium as a location where people could swim with the Obi-Amphibians (NA: Sky Pirates!) and another account refers to Elysium as being a lost Earth colony. (NA: Parasite)

The latter account also refers to the Elysium system as being a "lost colony of distant Earth" and the site of the Artifact. (NA: Parasite)

Earth colony
Elysium was an Earth colony. It was isolated from Earth and the Earth Empire for centuries. It had at least one, likely several oceans that produced severe tidal events. Elysium was initially populated when the colony ship crashed.

Civil war
When the Seventh Doctor, Ace and Bernice Summerfield encountered individuals from Elysium on the Artifact, the planet was on the brink of civil war.

The "Founding Families" ruled Elysium, they interbred with each other and stayed away from the general population. They favoured an isolationist system with a religious promoting large familiar/many children and sanctity and protection of life.

Whilst the the Reunionists, who had formed following the discovery of the Artefact supported attempting to contact the Earth Empire and break out of the closed system of Elysium. (NA: Parasite)


 * It is possible that, whether or not these are the same Elysium, that the Gates of Elysium existed on or in proximity to the aforementioned places. (DW: The Stolen Earth)