Pokémon X & Y will introduce Mega Evolution, a new transformation certain Pokémon can undergo during battle.
Pokémon such as Mewtwo and Lucario can utilise the new mechanic to alter their looks and increase their stats, giving Trainers an advantage in battle.
Pokémon that Mega Evolve will tap into hidden reserves of strength and power that have never been realised in their regular Evolutions, giving players battling with a Mega-Evolved Pokémon a dominant advantage. Mega Evolutions will revolutionise Pokémon battles and usher in a new Pokémon transformation never-before-seen in the Pokémon world.
Some Pokémon that Mega Evolve may even change their Ability or type while in battle. Only certain Pokémon, under certain conditions, can Mega Evolve, and unlike regular Evolution, Mega Evolution only occurs during battle-once the battle ends, the Mega-Evolved Pokémon will return to its usual state.