Narset, Enlightened Master

Oracle Text

First strike, hexproof

Whenever Narset, Enlightened Master attacks, exile the top four cards of your library. Until end of turn, you may cast noncreature cards exiled with Narset this turn without paying their mana costs.

Card Rulings

9/20/2014 The cards are exiled face up. Casting the noncreature cards exiled this way follows the normal rules for casting those cards. You must follow all applicable timing rules. For example, if one of the exiled cards is a sorcery card, you can cast it only during your main phase while the stack is empty.
9/20/2014 You can’t play any land cards exiled with Narset.
9/20/2014 Because you’re already casting the card using an alternative cost (by casting it without paying its mana cost), you can’t pay any other alternative costs for the card, including casting it face down using the morph ability. You can pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.
9/20/2014 If the card has in its mana cost, you must choose 0 as the value for X when casting it.
9/20/2014 Any exiled cards you don’t cast remain in exile.