"After a player initiates a challenge and declares a target and attackers, you may use this title to make that player choose a new target for the attack..."
Reading that, I have to wonder why I am asking this question - but it cropped up in a game last night and I lost the vote 2:1
Player A declared an attack on player B using Margery Tyrell (The one that can kneel someone to put them in the challenge as a defender)
Player B wanted to re-direct the challenge and was allowed to redirect it before any attackers had been declared ... i.e. to wiggle out of Margerys effect
I don't see how they can do that ... they could redirect but Margery's ability would be triggered first and a character would still be knelt and participating as a defender
Some issues have also cropped up with Crown Regent and Dragon Sight
Here I assume that the original target can just declare no attackers, and after the attacker has declared his attackers the challenge can be redirected ... at which point it is too late for the new defender to declare any defenders?