The Dark Council chamber is only seen during the Onslaught expansion playthrough if the player's base class is Imperial.
Despite the Dark Council being reduced to 5 seats, the chamber features 12 seats, not including the Emperor's throne, with 6 seats on each side.
If the player supported the Empire on Iokath, then Empress Acina would chair the meeting of the Dark Council during the Onslaught expansion otherwise it would be chaired by Emperor Vowrawn.