The Prism Between Light and Darkness!

After the mysterious creature known as UB Black absorbs Solgaleo and retreats into an Ultra Wormhole. Ash and his friends give chase and find themselves in Poipole’s original home. There, they meet a giant Pokémon named Naganadel, who tells our heroes of the threat to its world—and how UB Black may be the only hope of saving it!