There was never just one Arceus. There had recently been a new Arceus, who was meant to assist the first. Over time however, the once cheerful young Arceus woman gradually became more sadistic and hostile to the world she had lived in. Constant arguments from the legendary pokemon, the constant paperwork, and the lack of time to relax and cuddle with her beloved Reshiram girlfriend Flare wore down on her mental state until finally...
It opened.
A space-time rift above ancient Sinnoh - then called Hisui - opened, and a corresponding rift opened somewhere in an isolated mountain range. Whoever ventured into the rift would find themselves forcefully transformed into an Alpha Sexymon (female anthro pokemon) and then released into Hisui on Prelude Beach. Oftentime this ruined friendships and caused those affected by it much suffering and pain, both physically and mentally, as they felt themselves lose control and slowly be consumed by the feral personality forcefully implanted in them...
As more came, the time-space rift grew more unstable due to how many beings from another timeline appeared... and then came along a certain young couple, who, without knowing as they approached the rift, would work towards Hisui's liberation...