The Place of No Stars (informally known as the Dark Forest) is the opposite of StarClan. It is a place where cats who go against the Warrior Code and/or their Clans go when they pass. However, cats who have committed crimes but believe that they are following the Warrior Code can join the ranks of StarClan, for instance, Ashfur.
There is no prey and cats cannot feel hunger. Instead of being with friends and families, you walk the skies in solitude. Tigerstar seems to have found a way around this. Much like StarClan, the cats who are forgotten by the living fade to ghostlike spirits.