The general question is about the timing of stealth. When is it declared and take effect?

Specific examples

1. My opponent has Syrio Forel and initiates a military challenge. I bring out Ser Mandon Moore from shadows, does my opponent have a chance to stealth him before he can become a defender? Would it matter if I had any other characters out before Moore leaves shadows?

2. My opponent has Frozen Moat in play (Challenges: Kneel Frozen Moat to choose 1 attacking character. Until the end of the challenge, treat that character as if its printed text box were blank.)
I declare an attacker with stealth, does he have the opportunity to trigger Frozen Moat to nullify my stealth before it can be used?

Thanks

Take a look at the FAQ posted on the website. In there you'll find the framework for the challenge phase and it will answer all your questions (and others you might have).

Stealth is declared right before defenders are declared, and you cannot trigger any phase or challenges effects in between declaring stealth and declaring defenders.

So in the first case, you will always have the opportunity to stealth before declaring Ser Mandon as the defender, since you cannot trigger his ability in between declaring stealth and defenders.

With frozen moat, you will be able to trigger its ability (blanking an opponent's character) before stealth is declared, and thus he won't be able to stealth your characters. However, frozen moat can also be used after stealth and defenders are declared since there is another player action window in between declaring defenders and implementing the challenge results. By this time however, any character bypassed by stealth will remain bypassed. So it's up to you when you want to trigger Frozen Moat.

boben_fett said:

Yes to both of your questions.

The thing is that assigning stealth and declarng defenders happens in the same framework window so there is nothing you can do between but you can play actions after attackers are declared and before stealth is assigned.

1. declare attackers (he declares syrio)

2. action window (you can bring out mandor moore or trigger frozen moat)

3 stealth is assigned and defenders are declared (so Mandor is already in play so he can be stealthed, in the other case syrio does not have the stealth anymore due to a frozen moat)