So in my game, I have 3 cards left in the Archfiends deck. When they run out, should I shuffle the discard pile to make a new Archfiends deck or, since I've made it through all the cards, I get a break and stop drawing new cards?
I'm going to say that yes, you can catch a break.
I'm almost positive I said the opposite a year or two ago in this same forum, but looking through the cards and their effects...yeah.
Once you've survived the Archfiend's powers once, whatever's on the Archfiend card stays there to empower it and whatever's been discarded stays discarded. All that's left is to seal that last rift and handle your final showdown.