Nivix, Aerie of the Firemind

Oracle Text

T: Add C.

2UR, T: Exile the top card of your library. Until your next turn, you may cast that card if it's an instant or sorcery card.

Card Rulings

2/1/2006 “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.”
2/1/2006 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.
2/1/2006 If you don’t cast the card, it stays exiled.