Rules FAQ
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If an opponent initiates a When Revealed ability on your plot card "as if they had just revealed it" with a card like Varys's Riddle, Pulling the Strings, Weirwood Grove, Tyrion's Chain or Taste the Blood, it is considered to be an opponent's ability from your point of view, so you can cancel it with Someone Always Tells. Same if they play an event from your discard pile with Archmaester's Key.
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Unlike the original version of the card, this one can only cancel When Revealed abilities on plot cards. It cannot cancel other triggered plot abilities like The Mad King's Command (BtB) or Winter Festival.