This came up in our game and we were unsure of the proper ruling. Player A playing Hollow Hill flips valar. Player B flips at the gates. Player A, winning initative, chooses to play the "when revealed" on at the gates first. Here is the question. Player B has a maester in his deck. Does he have to put a maester into play from his deck, knowing the "when revealed" of valar will be played next, killing the maester from at the gates. Or can he choose to not put a maester into play?

He must search for a Maester and put it into play if one exists in his deck. If At the Gates said "When revealed, you may search your deck for an in-house or neutral Maester of 3 cost or lower and put it into play." then he would have a choice. Since "may" is not present in the current text, you must fulfill the "when revealed" if it is possible.

Thanks.

That is the way we played it.

The player who revealed At the Gates can search for a duplicate of a maester he already has in play or a maester who can save himself like Aemon or Wendamyr.