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.
Travel from world to world could be made through the Master's House, which appears to exist in all the worlds simultaneously, or by master runes from the Book Bound in Pale Leather. The remnant of the Kencyrath that survived the Fall used Anthrobar's Scroll, a partial copy of the Book, to reach Rathillien.
These worlds have been so far named in the series:
- Rathillien, the current world
- Ch'un, where "the very stones sang"
- Krakilleth, "golden"
- Lury, "green"