Since Ghaston Grey refers to the player who owns the card, it makes sense that the owner picks which participating character is returned to their owner's hand. But it does not mention choose in the card text. So, who chooses?

The text on Ghaston Grey is as follows:

" Reaction: After you lose a challenge as the defending player, kneel and sacrifice Ghaston Grey to return an attacking character to its owner’s hand (cannot be saved)."

Card abilities are always read from the point of view of the person controlling the card. The person controlling the card ability (in this case, Ghaston Grey's controller) is the player that does everything on the card, unless it specifically says otherwise. So the controller of Ghaston Grey gets to select the attacking character that is returned to its owner's hand.

The lack of the word "choose" only indicates that the attacking character returned to hand is not considered a target of Ghaston Grey's ability. It isn't some subtle, hidden meaning that allows someone other than the person triggering the reaction to pick the character hit by the reaction.

Thank you Ktom, I appreciate the quick response!

If Ghaston removes the only attacker does challenge resolution stop?

If Ghaston removes the only attacker does challenge resolution stop?

No, for two reasons - first, the challenge has already been won. Everything about the state of the ending of the challenge is set at that point (e.g. whether it is unopposed), even though it doesn't grant an UO power till later.

The second is that there isn't any effect int he game that stops challenge resolution, once a challenge has been initiated. Challenges don't fizzle - once initiated, they go through to completion. You could remove the only attacker with highgarden, or Dracarys, or Areo - but you'd still get the opportunity to block, potentially win, etc.

Ok cool, just wasn't sure because moribund has gone and Ghaston triggered in the win lose section of the framework, but if resolution carries on that's fine.