Dark Omastar

Prehistoric Water: 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. | Dark Tentacle (30): 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.)