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My explanation for everything was too long, so you'll just have to take things at face value.
Basically, this is a Sinnoh journey where somewhere inbetween, timey wimey shit happens so my character accesses Hisui, then goes back to Sinnoh.
Regidraco is a Pokemon that was required to awaken Regigigas, which Team Galactic was planning to do, so an arc involving catching the 5 Regis before Team Galactic can was initiated.