Once you finish the game, go to Lumiose Station, and the Professors Assistants, AKA, the Masked Heroes, will give you a boarding passed. You will then go to Kiloude City. Find a building called the Battle Maison. Challenge it once, then look around Kiloude City. You will then find your rival, Calem or Serena, depending on which gender you chose. Battle him or her, defeat her, and they will give you an Absolite. Then as for extra info, after that, go to the sundial in Anistar City, and you will be able to Power Up your Mega Ring, to where you will be able to detect mega stones at 8PM to 9PM.