As long as any Stadium card with Holon in its name is in play, your opponent can't play any Stadium cards from his or her hand.
Choose 1 card from your opponent's hand without looking. Look at the card you chose. If that card is a Trainer card, this attack does 30 damage plus 30 more damage, and discard that card. If that card is not a Trainer card, return it to your opponent's hand.