If you have cards in play with an ability that reads
"You may initiate an additional ... challenge during the challenges phase."
what happens if some of these cards leave play?
Option 1:
When initiating the challenge you choose which challenge it is.
Option 2:
When initiating the challenge you must have enough cards in play to allow the challenge.
Example:
1. You have Khal Drogo in play
2. You initiate and resolve your first military challenge
3. Your opponent kills Khal Drogo
Are you allowed to initiate another military challenge, because you say the first challenge was the additional challenge from Khal Drogo and you still have your one default challenge?