Buoysel

Constantly moving throughout the ocean the Temple of the Sea exists invisible to human eyes. It is here that the Sea Crown exists. Phantom the Pirate has captured the egg of a Manaphy, the only Pokémon able to locate the temple. However, his plans are ruined when Jack Walker, a Pokémon Ranger, interferes. Along the way, Ash Ketchum and his friends manage to get caught up in…

Dialga rules time, and Palkia rules space, they lived in an alternate dimension so they decided to fight within their own reality, which would make it bad for the real world too. Dialga and Palkia appear before Ash and his friends and drag the city into their own world. If they destroy this city, they'll also destroy the whole world. Source: ANN