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In the pre elite4 story Corrina or the regional NPC gives you an Imperfect Megastone wich it requires you to win 5 battle befores you can use it, when you mega-evolve the stone lost its energy and you need 5 more wins against trainers to activate the mega-evolution again.
Once you reach the end of the victory road some NPC will gives you a perfect Megastone wich you can use whenever you want
Also all the elite4 + champion can megaevolve.
