Hyperspace Lumiose is a post-apocalyptic version of Lumiose where the new Ultimate Weapon in Prism Tower (Ange) was successfully activated, and the city was devastated

This also led to the creation of new mega evolutions, not found in the regular Lumiose, like Mega Raichu X and Y

You have to find and befriend this world's version of AZ's Floette to awaken Mega Xerneas/Mega Yveltal (the one you get depends on the choices you make during the game) to restore Hyperspace Lumiose