The Shadowfell
Little is known about this location which seems to be another plane connected to ours through Antaeus.
Rhea Mythorious has warned that certain spirits had been twisted by an entity during the long night, and were too powerful and damaged to be left to their own devices or banished. So they were chained. In the days of the druids, they would often tend to these spirits, mending them if they could or imprisoning them inside the tomb of Idryin (check spelling) but the entity corrupted and broke the druid circles of old, and so Rhea and her kind have done what they could in the millennium since. This seems to be the way in.