I played as Martell, a Friend of mine as Stark.
He attacked me with military and I didn't defend. When the challenge resolved, I played red vengeance and named him the new target to fullfill the claim value.
So in my opinion he would be both, winner and loser of the challange, because he is the new defender. He said, I still lost the challenge. Does he get a Power Icon for undefended, because i did not defend? If i really lost the challenge, can i trigger the abillity of Areo Hotah?
A Side question: Is Deadly, or stealth a character abillity, so the red Viper would be immune to that? Or are character abillities just keywords like: Response: Challenges: and so on...?