Caer Preavdyn is the Castle Between the Planes, an area of Wild Magic located in the far Western part of Darkhold. It is said that the terrain there is variable, and changes constantly. It is said that, for a price, you can travel from Caer Preavdyn to any plane. The castle keeper collects the fee. Few people are known to have ever made it to the castle - it tends to be an act of desperation.