What remains of the former Prison Planet.
Part of the Cluster
The Prison Planet
During the time of Cryos's forefathers, they and the other planets' rulers established this planet as a prison to house the Cluster worlds' deadliest criminals. Cell blocks were constructed on the surface that were protected by force-fields to sustain an artificial atmosphere. It was then dubbed the Prison Planet and used solely as such. Femur sent his usurped brother, Sternum, here.
Riot and Destruction
Eventually, Sternum lead a huge riot among the inmates during which he stumbled upon the Teleport Engine; an alien device similar to the World Engines that were later discovered within the other planets in the Cluster. He used it to teleport the entire Prison Planet away from the Cluster. After being teleported, the Prison Planet sustained heavy damage and was destroyed, leaving only a small piece of the planet, and only one cellblock, intact. That piece contained the Teleport Engine so Sternum could continue to teleport them. Sometime later, a crystaline alien named Jewelia crash-landed on the shard and became the leader of the two opposing factions that began fighting for control of the shard and, by extension, the Teleport Engine within.
The Beast Planet
The shard was eventually teleported directly in the Beast Planet's path, resulting in contact with it's Claw. The Teleport Engine was then activated by Sternum, teleporting it, along with the Beast Planet, to an unknown location. The shard was destroyed in the process, killing all who remained on it, as they were most likely consumed by the Beast Planet. Only Sternum, Jewelia, Jade, Graveheart, Femur, and Cryos survived its destruction, as they had teleported to Planet Bone before the Prison Planet teleported away.
The planet contains members of each race of the Cluster, including Rock, Bone, Fire and Ice. The only member of a different race is Jewelia, who claims to have crashed into the Prison Planet and become stranded there as Graveheart and Cryos did, but she bears some similarities to the people of Planet Rock.
Rats. (Or at least creatures who sound like rats, as rodent-like squeaking was heard in the prison cell were Femur was briefly imprisoned.)