This is an absolutely key card for House Lannister and can really ruin your opponent’s day! Cancel effects are rare at the moment and House Lannister gets the trump card! Given that triggered abilities are often game-winning in terms of strength, the ability to cancel is clearly invaluable.
As an example of the card’s power; recently I played a very fun game against The Night’s Watch. After winning an intrigue challenge I played Tears of Lys on Old Bear Mormont. My opponent then sacrificed Bran Stark to cancel the effect. Out comes Treachery and poor Bran dies in vain along with Old Bear Mormont!
Of course there are a multitude of nasty cancels in line with the above. Make sure you know your opponent’s faction and save this card for the really powerful effects, for maximum morale destruction!
The only downside of Treachery seems to be that it does not cover plot effects or events. But hey, no card can be perfect!