The Primordial Realm is a dimension different than Gielinor, which can be visited during the Stealing Creation activity. It is believed to be the place where the gods retrieved the materials to create Gielinor, even though Guthix apparently made Gielinor through the Stone of Jas.
As there are no restrictions on combat in the Primordial Realm (other than the fact that you cannot attack members of your own team), many adventurers enjoy taking the battle more seriously by attacking their enemies as they gather resources, build items, and so on. This is also a good way of gathering clay, as any foes you defeat will drop all of their items (unless they have a Protect Item prayer active), which you can pick up and return to the tables in your base.
There are also many Fog Banks found in this realm, in which one may completely conceal oneself.
The primordial realm is a flat and basic place filled with fog banks and sacred clay resources. At the corners of the primordial realm are bases where players can deposit sacred clay and sacred clay items.