|Garde Manger Guy|
Barghests love to consume corpses. The fresher the better. Even though Malfeshnekor is a greater barghest, he still hasn't eaten in a very long time. It would be well within Nualia's character to say and do this.
"What just happened here? The mighty heroes of Sandpoint thought they could just stroll into our complex and defeat us? We all see how that went. Useless scum! Let us not let them off so easily by burning them now! When I free Malfeshnekor, his first meal shall be the flesh of these so called heroes and everyone here can witness their destruction!"
The party is then stabilized and thrown into the dungeon until Nualia can open up the prison which takes... however many days (less than a week though) you need until a "rescue party" can come get them. Several NPC's in town have enough levels to mount a commando style raid with the sole intent of rescuing the PC's. I would let the party members run them so they have something to do during this time. It would also give the PC's a chance to learn the abilities/ increase the levels of the NPC's for the giant raid down the road.
This part is NSFW. It will contain thoughts and magic items that are mature and disturbing in nature. Parental discretion is advised.
Malfeshnekor is released by this act and immediately leaves to recruit the Goblin horde. Nualia leaves to have a child that would appear later in the adventure but not before telling Ripnugget to "Finish my light work" in true Evil overlord style.
The "rescue party" would have a much easier go of it in this case, and the PC's (who still need to recover after their ordeal) would be ready to deal with Malfeshnekor's raid in about a month give or take.