The Place of No Stars, or the Dark Forest, is the place where dead cats who were rejected from StarClan reside.
The cats of the Dark Forest are currently training living cats in their dreams so they can later attack the actual clans, this will be called the Final Battle. It is currently unknown when the Final Battle will be. It should happen sometime in the Last Hope.
The Place of No Stars is described as being a forest with no sun, stars, or moon, so no light at all. It is said that the eyes of spirits long forgotten gleam under the bushes. It is also said that they have a river that is not water, but more like slime. It is mentioned and shown in the manga in Crookedstar's Promise that there is a border in between StarClan and the Place of No Stars.
List of Known Members
Mapleshade (fades away)
Brokentail (spirit later destroyed)
Tigerstar (spirit later destroyed)
Hawkfrost (spirit later destroyed)
Antpelt (though his spirit was later destroyed)
Hawkheart (mentioned by the authors that he most likely went to the dark forest)
Shredtail (spirit later destroyed)
the Dark Forest hosts more unknown spirits.
List of Dark Forest Visitors
Ivypool (working against them)