Alhammarret, High Arbiter

• WWizards of the Coast2015-06-22
  Official Ruling
  You choose one card name, not one name per opponent.
• WWizards of the Coast2015-06-22
  Official Ruling
  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.
• WWizards of the Coast2015-06-22
  Official Ruling
  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.
• WWizards of the Coast2015-06-22
  Official Ruling
  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.
• WWizards of the Coast2015-06-22
  Official Ruling
  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.

Rulings via Scryfall