What do you guys think the priests will do with the surviving party members? I'm trying to come up with a scenario that gives them a chance of breaking out.
It depends a lot on your party - both the players and their characters. Are your players the sort to come up with a cunning escape plan? What abilities do they have that would allow them to break out?
The obvious place to lock the party up is in the chamber of the Beast - it's quite large, and already has a chain and padlock. The priests might well want to question the party members to find out who they are and why they came charging in to the citadel. So I'd have them throw the party in Beast's chamber (after stripping them of their dangerous gear), lock the door, then go away to lick their wounds. This will give the players a chance to discuss their options. After some time (depending on how long you want to give them to prepare), the priests and tiefling guards will return to take one of the PCs away for interrogation. This pattern will continue, as they try to corroborate the story they're being told.
An alternative would be for Theldrick to hand the party over to the authorities in Diamond Lake, or the Garrison - since their murderous raid on his citadel is clearly illegal, while the Hextorites themselves are very law-abiding. :-) It all depends on how you want to play it out, and what will work best for your players!
I had this scenerio where two of the characters got captured. I had switched things around so that the enterance to the mine (elevator) led to the Temple, and then on from that it led to the room with the pool (the temple is the first line of defence to the inner central cathedral).
When they eventually wiped out the temple they found their comrades hangs above the pool with their blood dropping in.
Just to be really evil I had them infected with Slow Worms in the hope that the worms would either break out in one of the the other temples, or even better in the town above (inspired by the film Relic).
Heh, when my group's party got nearly TPK'ed - most were only knocked out; but the fire-cleric died, had a close encounter with his fire-god's agent (Gendinom, from a Wormfood article) who intercepted his soul in the afterlife and agreed to bring him back to life in exchange for a unique creature to throw into his furnace, to be delivered at some sooner-rather-than-later date in a ritual which (coincidentally?) cost the same as a Raise Dead spell - after blundering into the Hextorite 'kill-room', I did some fast thinking between sessions and had them all wake up in a cell I had added to the map near the boss's rooms (never thought of using Beast's room - great idea). With them was a spy the hextorites had previously discovered and captured, who turned out to be a fanatical assassin from Hextor's 'secret police', who had been tasked with infiltrating this blasphemous sect and wiping them off the face of Oerth for their unspeakable heresy. He had been discovered while playing the part of one of the lowly cultists, and was being kept for a sacrifice in some upcoming ritual (ie. awakening the aspect). As was the party. Together, they managed to overpower a guard and escape the cell. The assassin took the guard's uniform, pulled his hood up to hide his face, and used it as a disguise to get close to the boss and managed to hit him with a death attack while he was preoccupied with the escaping PC's - the lost XP and gory glory for offing him was the price the party paid for needing help. Once they'd mopped up the remaining Ebon Hextorites, the assassin went off on his way and hasn't been seen since. They've been much more careful about blundering headlong into kill-rooms ever since, and the fire-priest had a between-sessions mini solo-trek to the circle of standing stones at the Bronzewood Lodge, where he carried the Ebon Aspect's body, at the bidding of a fireside vision, to deliver it to Gendinom (gated in by a Bronzewood elder who also had a vision) for cremation. A tough but random attack from a wolf pack on the way there earned him back some of the XP he lost for dying, and almost cost him his delicious horse to boot.
All in all, though it may seem like I went a little easy on them (especially on the cleric, what with the Raise Dead on IOU while imprisoned with all of his allies), I think these were some of the more memorable sessions so far in the campaign for my players (I suppose Mr. Mearls probably deserves some of the credit for that, having written this excellent adventure and all... :) ) IMO, well worth the small amount of DM-fudgery it required; I think it's the only time I've ever really succeeded in making the gods of D&D feel like something special and important, beyond whatever time of day the party cleric prepares his spells, of course... And all this came up spontaneously, from what would otherwise have been a campaign-ending disaster! Can't guarantee it'd fly in anyone else's campaign, though; YMMV. Some players and DM's I know would scoff at my handling of the situation and tell me I should have let them all die instead of mollycoddling them. To me though, as a DM, it was great - saved my campaign from certain doom, got to introduce a new NPC (Balgon the Eunuch - the assassin); got to make use of Wormfood and related super-powerful extraplanar ally; got to pimp-slap the whole party, so to speak, and make the players sweat for a week wondering what would become of their characters; even managed to find a use for the Bronzewood Lodge! Got to DM a solo adventure too. To me, that's a lot of firsts and high points for just one or 2 sessions.
So you see, don't despair - it might seem like a total disaster right now... but if you use your imagination, you never know; it might just turn into one of the high points of your whole campaign. It did for me.
Good luck!
Kang