ambush keyword is a passtive effect

db123456 said:

Are you sure ?

Someone who consider himself an anthority tell me ambush is a trigger effect , so that when Brienne of Tarth is participating the challenge , I can NOT put a card into play by "ambush" ? Is he right ?

From the core rules:

"You may put a card with the “Ambush” keyword
into play from your hand as an “Any Phase:”
action, by paying its printed gold cost with
influence."

Since it is played as an "Any Phase:" action, it is a triggered effect.

Edit: I do not consider myself an authority, but the rules are clear on this.

Keywords are not passive by definition per se, and can be constant or (in the case of Ambush), triggered. However, it cannot be canceled (by definition in the FAQ; keywords cannot be canceled).

Ambush is triggered not only because the definition of it provides that "Any Phase" trigger but because it happens only because the player chooses for it to happen. And, since Brienne does not actually cancel anything (but prevents them from being triggered in the first place), she stops players form using Ambush as surely as she stops players from using "Any Phase" events.