Oracle Text

As Nevermore enters the battlefield, choose a nonland card name.

Spells with the chosen name can't be cast.

Card Rulings

9/22/2011 No one can cast spells or activate abilities between the time a card is named and the time that Nevermore’s ability starts to work.
9/22/2011 Spells with the chosen name that somehow happen to already be on the stack when Nevermore enters the battlefield are not affected by Nevermore’s ability.
9/22/2011 Although the named card can’t be cast, it can still be put onto the battlefield by a spell or ability (if it’s a permanent card).
9/22/2011 You can name either half of a split card, but not both. If you do so, that half (and both halves, if the split card has fuse) can’t be cast. The other half is unaffected.
9/22/2011 If you want to name a double-faced card, remember to name the front face of that card. (The back face can’t be cast anyway.)
9/22/2011 The named card can be cast again once Nevermore leaves the battlefield.