So, if there are so few of them left alive and their leaders are dead, why do the survivors even have any morale left that would enable them to stay there? I would think the lowest would flee, while maybe a couple of others would think to retreat and go for backup rather than just mindlessly run. That would leave only a few more still there. It does not matter how Lawful the race may be, Evil is very cowardly when solo or in very small numbers, especially when they know the ones who just trashed their forces will emerge at full power in mere hours, or even minutes, if they have no clue how much time has already passed since the party holed up in the leader's rooms.
Well, that's one of the reasons I posted. I can easily see the survivors thinking this isn't worth the bother, cut their losses and return to the Underdark. Then again, one or both of the surviving overseers might have enough resolve to rally the remaining forces and at least make a show of strength or make the effort to bury the PCs alive before withdrawing.
But it does seem like the duergar position in the stronghold is untenable. They haven't got enough manpower to hold the fort now, and reinforcements are probably weeks away. So it might very well be more logical for them to simply abandon it and go back to the Underdark ... with perhaps a long-term goal of rallying more troops assault the hold again and hopefully keep it.
It was their durzagon leader's idea to ally with the Red Wizards. Perhaps the surviving duergar are not so keen and would rather stay alive than risk their lives for some pasty-faced weirdos.
How does this sound?
1) Once the surviving duergar figure out the situation, they come and test the PCs' defenses. When they realize it would potentially be suicidal to try and assault them in the barricaded room, they decide to bury them alive by collapsing the ceiling in the corridor. But they have the courtesy of telling the PCs this by sending an interpreter, who can reveal that the duergars' slaves are the missing villagers from Julkoun. The interpreter will warn them that if the PCs let themselves be buried alive, then the duergar will most likely take the slaves off into the Underdark and they'll never been seen or heard from again. (I might have the interpreter tell them that there less than a dozen duergar left as well.)
That way it will be a hard choice for the players: Do we stay in here, rest up, and then have to dig our way out and lose the villagers, or do we take the risk and bust out now and hope we can defeat the remaining duergar before they defeat us so we can potentially rescue the slaves and take them home to their village (which the PCs previously liberated from the hobgoblins and is now being watched over by allies)?
Perhaps the villagers might even want to help the PCs. Maybe they make a distraction to help the PCs get out of their hole without having to face a full duergar ambush. (The adventure as written does suggest that, if the PCs invade the mines while the villagers are still there, they will help the PCs by distracting their duergar guards by grabbing at their feet and otherwise getting in the way; on the flip side, it also says the duergar are not above using them as human shields ....)
