As Alhammarret, High Arbiter enters the battlefield, each opponent reveals their hand. You choose the name of a nonland card revealed this way.
Your opponents can't cast spells with the chosen name (as long as this creature is on the battlefield).
6/22/2015 Alhammarret’s second ability happens as Alhammarret enters the battlefield. No one can cast spells or activate abilities between the time a card is named and the time that Alhammarret’s last ability starts to work.
6/22/2015 You choose one card name, not one name per opponent.
6/22/2015 Although spells with the chosen name can’t be cast, permanent cards with that name can still be put onto the battlefield by a spell or ability.
6/22/2015 If your opponents have no nonland cards in their hands, you can’t choose a card name. Alhammarret’s last ability won’t stop any spells from being cast in that case.
6/22/2015 Your opponents can still cast a card with the chosen name face down if it has a morph ability. A card with the chosen name can also be manifested.