My players made it to Bozal, and in the process of fighting him, managed to blind him for a round. He was down to 9 hit points (I had given him max hit points) so he lost his nerve and released the Ulgurstasta early.
My players currently cannot enter room 28 because Bozal has an anti-life shell on. What should Bozal do? He still has Darkness avialable to him. He still has several spells up, he could cure himself.
Then there is the other problem, the Ulgurstasta. Clearly it doesn't see anything to attack. So it should start attacking the ceiling.
I am not quite sure how I should handle this. Would Bozal do his best to attack the players? Would he sacrifice himself and allow himself to be eaten by the Ulgurstasta? Would he cast darkness and flee into the secret passages beyond?
The Ulgurstasta is pretty straight forward. It will attack the players if it sees them, if not it will do its best to burst out of its room into the Arena above. If that happens I imagine the games may very well be over. What would Raknian do if that happened?
I would say that he would do Darkness then cure himself a bit. Then try and lead the characters to the Ulgurstasta to try and get then to attack it. Then he will attack them at the same time hoping that the two of them can take them out. Maybe a Hold Person on a character so the thing can swallow them whole and spit them out.
If the players haven't already encountered the Alkilith demon you could also bring him into the fight.
I modified the module a little bit in case something like this would happen. Have the ulgurstaasta burrow down into the ghoul area instead of up into the stadium. This allows the PCs to follow him (eventually). You could also leave the impression that the ulgurstaasta has fled and the players are victorious, only to have him return during the final battle thanks to some type of behind-the-scenes action from Raknian.