Does a card that triggers when playing cards face-down activate when playing cards from deck (like Nocturne) or with Rek'Sai effects?

Ruling: No, cards in the deck are not considered "face-down." Face-down specifically refers to cards on the board with their back side presented. Cards played from deck (like Nocturne's banish effect) will not trigger effects that require playing cards face-down.

Nuances:

• Banish is not a face-down zone
• Cards in deck have their back side presented but are not technically "face-down" - that term only applies to cards on the board
• The "face-down" wording is likely future-proofing for cards that may be played face-down on the board in future sets