We are about to witness some eldritch scissoring

My theory is that the people that originally made Magiana had followed/worshipped Diancie. But one day she disappeared and wasn't heard from again, so some of those people wanted to create a "new Diancie" to take her place. With some spare parts and help from a horde of Klinks to help them, they created Magiana.

But the two groups, those who helped make Magiana and those who stayed faithful to Diancie, developed animosity toward each other and have almost wiped each other out in war over the decades that have passed. So Magiana, the replacement Diancie, left them and wanders the world to find the original and try to bring back peace. Diancie would refuse to help her "lesser replacement" out of spite for her followers being taken from her, and the battle begins.

As for Volcanion's involvement? He would be the only one to know how to get Diancie and Magiana on the same page so they could bring peace back.