Dark Omastar

If your opponent has any evolved Pokémon in play, choose 1 of them and flip a coin. If heads, your opponent takes the highest Stage Evolution card on that Pokémon and shuffles it into his or her deck.

During your opponent's next turn, the Defending Pokémon can't evolve except from effects of attacks or Pokémon Powers. (Benching that Pokémon ends this effect.)