They are an old wives' tale that if you find a cold spot in the woods/forest, it is a breach in the planes and may send you to the Shadowfell or Astral Sea.
A place that borders the astral plane but connects directly into the Shadowfell. It is invisible, so you can't see it unless you are in-tuned with the plane of dreams, the astral planes, or are capable of True Sight. It was believed that anything which touched a ghost circle ceased to exist, however, thanks to planar travel and the Guardianship of Kalashtar tribes, it is discovered that those which touch Ghost Circles are shunted into the Shadowfell and are trapped in a ghost-like state.
This forges interaction between Kalashtar and Unknown. Kalashtar, being tribesmen in-tuned with the Astral are able to, at some point, gain enough power to fully close these ghost circles. Shadar-Kai happen to use these pinpoints as portals to walk onto the material plane or, in dire or dreadful circumstances, learn to weave these Ghost Circles themselves or even worse - force them to mutate to go not to the Shadowfell, but to the negative plane where things truly DO disappear. This has forged a rivalry between Kalashtar and Shadar-Kai, as one is a being lacking emotions, dreams, and whatnot while the other is directly linked to a Dream Being.