Following the culmination of the battle between Dariat, Rubra and the possessed for control of Valisk, the combination of the two affinity-cabable humans shifted the habitat into a realm that would be actively hostile to the possessed.
The dark continuum proved to be inhabitated by the Orgathé creatures that desperatly sought energy… and it was very difficult for Valisk to survive. A function of the dark continuum was that it prevented energy from forming within the region, thus preventing escape. Dariat likened it to a lobster pot.
In the online FAQ, the author suggests that the dark continuum is the same thing as the ghost realm. This does not, however, seem likely, although it is possible that the ghosts might end up in the dark continuum.