A threshold world is a world in one of the parallel universes called the Chain of Creation that overlaps with worlds in two other of the universes so that passage can be made from one to the other. It is through threshold worlds that Perimal Darkling has entered universes to devour them, and it is on threshold worlds that the Kencyrath has fought their long series of retreats. Rathillien, the world on which the Kencyrath now resides, is a threshold world.