Cocytus

The Cocytus (コキュートス) is an area connected to all Dark Areas, with each Digital World having its own Dark Area. There is only one Cocytus, with the location shared by every Dark Area, with Cocytus acting as the deepest layer within each Dark Area, connecting them all to each other. GranDracmon, Gulfmon, and the Seven Great Demon Lords have made the Cocytus their bases of operation.

Digimon Chronicle X
The take residence in the Cocytus. had wait in the Cocytus and after the Demon Lords were defeated it teleported their bodies to the Cocytus and then had Keramon warp digivolve to  and fuse with the bodies of the defeated Demon Lords to create.