History
When the Multiverse was destroyed in the last incursion between Earth-616 and Earth-1610, Doctor Doom, with Beyonder's powers, recreated it in the form of Battleworld. Earth-15730 was recreated in the Westchester domain, ruled by Baron Kelly.[1]
The Shadow King realized that in comparison to those other domains, Westchester was a fragile paradise, and set out to ensure its survival by any means. Using the cloned body of Professor X, the Shadow King became Cassandra Nova, and used his new identity to become a politician and establish the Bureau of Super-Powers.[2]
At the Clear Mountain, Cassandra Nova used a form of psychotherapy called the "Mind Field" to brainwash those mutants who arrived to the institute and force them to live in harmony, unlike most of the other mutants, Leech and a few other more monstrous looking mutants were kept in a separate room and called the Rej-X, deemed not fit to integrate into the human society because of their looks.
Jubilee who didn't take part in the treatment because she was too young, was led to them by Artie Maddicks.[2] Beast who was also sent to the room because of his looks found out from Chamber that the Rej-X weren't just trapped in that room doing nothing, they were forced to work by salvaging sentinel parts for Cassandra Nova Ten-Sentinel project.[3] Beast decides to build a cannon to bust them out of that room using sentinel parts and Chamber powers, but before they could test it, Deadpool who was looking for Sabretooth showed up and freed them.[4]