How does Nocturne's ability work when you look at cards in your library, particularly regarding timing, casting restrictions, and whether you need to reveal it to your opponent?

Ruling: When an effect allows you to look at cards in your library and you see Nocturne, you fully resolve the looking effect first, then Nocturne's triggered ability triggers. You may cast Nocturne only on an empty chain (not as a reaction). If you don't cast it, it stays where it was in your library.

Sequence:

• Step one: Fully resolve the effect that allowed you to look at cards (including effects like blue Teemo damage or Promising Future spells)
• Step two: Nocturne's triggered ability triggers when you see it
• Step three: You may cast Nocturne (only on empty chain)
• Step four: If not cast, Nocturne remains in its original position in your library

Nuances:

• You can have triggered abilities from cards your opponent cannot see
• The trigger happens when you see Nocturne, but the card does not explicitly state you need to reveal it
• There is an unresolved question about how the opponent can verify you're not missing the trigger to conceal information