(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.)
At the beginning of your upkeep, exile the top card of your library. You may play that card this turn.
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(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 rather than pay the mana cost for a spell you cast from exile.