This is a question specifically pertaining the hero card "Prince Imrahil" and the shadow effect on the treachery card "sleeping sentry".
Prince Imrahil has the ability to unexhaust every time a character leaves play and the shadow effect reads: "Defending player must discard all exhausted characters he controls"
Now since all these characters would be discarded from play by the same effect (also leading to the attack being undefended, but that´s another matter), I would assume the first player would be allowed to determine the order in which this happen (keeping in line the FAQ on simultanious effects timing). Thus it could be determined in which order the characters leave play. If so then when the first character leaves play, Prince Imrahil would be allowed to unexhaust.
The question now is: Should Prince Imrahil in this case still be discarded?
1) Yes, since he was exhausted at the time the shadow card was revealed. The fact that he is no longer exhausted doesn´t change that.
2) No, he is no longer exhausted and thus avoid the original shadow card effect
Any and all thoughts are welcome