C Common D/D/D Human Resources X TRAP Shuffle 3 cards into the Deck (any combination of "D/D" monsters in your hand, field, or Graveyard, and/or "D/D" cards in your Pendulum Zone), then you can add 2 "D/D" monsters from your Deck to your hand
Like most Pokmon cards, the value of Cloyster depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Fossil)
Your opponents get to trigger the attack abilities, which makes it an interesting card that plays incredibly well in EDH because your opponents need to consider how they attack (or whether they shoot Aurelia)
Ramping into Ulamog, the Ceaseless Hunger or Emrakul, the Promised End used to be quite potent against control strategies, and you can certainly imagine a world in which Field of the Dead alongside these threats pushes control decks out of the format in an unhealthy way