Nivix, Aerie of the Firemind
: Add 
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, : Exile the top card of your library. Until your next turn, you may cast it if it's an instant or sorcery spell.
Niv-Mizzet's genius and vanity reverberate throughout the mirrored halls of Nivix.
Illustrated by Martina Pilcerova
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"Until your next turn" means the effect wears off as soon as your next turn starts, even before you untap. It's basically the same as "until the end of the turn of the player who immediately precedes you."
The card is cast using all normal rules and restrictions for casting spells. Casting it means you'll have to pay its mana cost, and it will go to the graveyard after it resolves or is countered. The only thing that's different is you're casting it from the Exile zone.
If you don't cast the card, it stays exiled.
