At the beginning of your upkeep, exile the top card of your library. You may play it this turn.(You may cast either half. That door unlocks on the battlefield. As a sorcery, you may pay the mana cost of a locked door to unlock it.)Once each turn, you may pay {0} rather than pay the mana cost for a spell you cast from exile.(You may cast either half. That door unlocks on the battlefield. As a sorcery, you may pay the mana cost of a locked door to unlock it.)