So I know what you’re thinking: the other Sorshen has known the PCs were in her complex ever since the first encounter with the incubus (when it teleported away, and presumably warned its mistress). Why is she letting them approach her lair in such a leisurely fashion? Why didn’t she gather up the Gray Maidens and hit them en masse as soon as she learned of their incursion? Well, maybe you aren’t thinking that, but I was, in each of these sessions.
First, the gamey reasons. One is because it’s a genre trope: The overconfident villain sends small groups of minions against the heroes and allows them to fight her numerically superior forces in manageable numbers, leading to an exciting adventure that gradually builds up to a climactic battle that wouldn’t happen if the bad guys just overwhelmed the heroes at the beginning. Gotta follow the genre rules!
Related to this is a second reason: I don’t want to unfairly murder my PCs. Yes, I want them to be challenged, but I also want them to have a chance. A very logical thing to do would have been for the other Sorshen to show up with all her minions right when the PCs were recovering from their first battle with the incubus. She almost certainly would have killed or enslaved all of them, but that’s not a challenge: that’s the GM cackling maniacally as he engineers a near-certain TPK, and that’s not the type of GM I want to be. That said, maybe I was wrong. Maybe I should have sent the other Sorshen against them after all. I certainly know other GMs that would have, and maybe my players would have seen it as an enjoyable challenge rather than just the GM being unfair. I’ve been thinking about it a lot, and maybe next time I’ll do things differently.
But whether the decision to hold off was right or not, I had to come up with an in-game reason for it to happen, and I fell back on the following text in the module:
Unfortunately for [the false Sorshen], the curse also tends to make her easily distracted, forgetful, and capricious.
From this I inferred that she learned of the intruders, but was too distracted tormenting Orianna and selected Gray Maidens to take action herself. She instructed Danzomir, the incubus, to take the other Maidens and handle it without bothering her further. Danzomir was delighted to take command of the Maidens, who until that point had been considered the Mistress’ sole property. Being chaotic, he abused his newfound authority to engage in various assorted deviant behaviors of his own, and ended up wasting a great deal of time which a more dedicated commander would have used to organize the Maidens and more effectively repulse the intruders.
Damn, now that I wrote it out it seems intelligent and reasonable, not BS at all.