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They are obtained by Mega Evolving Pokémon-EX; doing so takes the player's entire turn, meaning that they cannot attack during the turn they Mega Evolve a Pokémon. Mega Evolution Pokémon have high HP and extremely powerful attacks.
The XY expansion will have two Mega Evolution Pokémon and six new Pokémon-EX cards. Four of these new Pokémon-EX have been revealed to be Venusaur-EX, Blastoise-EX, Xerneas-EX, and Yveltal-EX.
Four designs of the booster packs for this expansion have also been revealed, which feature Mega Venusaur, Mega Blastoise, Xerneas, and Yveltal.