The first thing that leapt out at me about this adventure is the fact that the main villain's plan is destined to fail. A succubus has infiltrated Sigil and she has two goals - steal the titular Harbinger House by causing it to shift into the Abyss and complete an elaborate murder ritual in the presence of the House's planar anomaly which will cause her to ascend to godhood. The first goal won't happen because the Lady of Pain has the absolute power to decide what enters or leaves Sigil and that includes mysterious creepy houses. The second goal is impossible because the succubus is missing a key component of the ritual - she's not a whimsically described "crazy" person who possesses a poorly-explained "spark of divinity" (an odd thing to say about a literal demon, but whatever).
So the actual goal of this adventure is a bit different than what the PCs will think the goal is supposed to be. They think they're rushing around to stop this Dark Ascension and the subsequent turmoil it will introduce into the Great Wheel's balance of power, but really the villain's plan is going to implode on its own, so all the PCs are really doing is mitigating the damage it does along the way.
This isn't necessarily a bad idea for an adventure. There's a Law vs Chaos theme at work here and you could make something out of that. This demon is stirring up shit that you're constantly having to clean up, and it's all completely pointless. That the demon's goal is doomed actually adds to the threat, because it means that everything that happens is totally absurd. I can see how it might provoke a motivating outrage among PCs with the right mentality.
But if you're going to do that, I think you need to be upfront about it. You don't want the players believe the game has a certain level of stakes, only to have the rug pulled out from under them. "Great job, guys. The main plot would have turned out exactly the same if you hadn't been there, but there are some minor NPCs who are still alive, thanks to you." Kind of takes all the steam out of the climax.
Plus, if you made it clear from the beginning that the succubus was destined to fail, you could play the revelation that the secondary villain does have the "spark of divinity" as a twist. See, she let a serial killer out of an asylum in order to cause a distraction while she prepares for the ascension ritual, but the killer has decided that he should be the one to become a god, so he's been incorporating the ritual into his murders. When the time comes for the final sacrifice, the succubus and her pawn fight over who gets to impale themselves on the blade infused with planar energy and become a god. If the succubus does it, she just dies. If the killer does it, he becomes a new god of murder.
That would be a fascinating scene for the PCs to get involved with, except the only way they ever learn any of this is the hard way. They don't become aware of the true stakes of the adventure until it's too late to make decisions based on that knowledge.
The other structural problem with this adventure is the way it has the PCs tag along behind the main plot. They start investigating this rash of strange murders and are always one step behind the killer as he continues his deadly work. This is preordained by the book itself directly addressing the DM - "However, while the PCs should be made to feel as though they had a chance to stop the murderer, under no circumstances should they be allowed to kill him or prevent him from escaping through the portal." This is in addition to all the times in the adventure where it says that if the PCs manage to save one of his victims, he just kills another person later on.
The funny thing is that, between the killer's savings throw and spell resistance, a Hold Person spell has about a 25% chance of working, and there is absolutely no DM handwaving that's going to get him out of that jam.
So I guess I'm saying that I didn't really enjoy it. The actual Harbinger House is a pretty neat location. It's filled with mysterious doors and some extremely odd people that the Believers of the Source think are close to becoming gods. It also includes magical phenomena that don't precisely fit the AD&D rules, presenting a slightly wilder brand of fantasy than is typical for the line. At one point, a corpse starts talking to the PCs, completely unprompted, to warn the PCs about the perils of the gate town, Curst. "Things happen in the multiverse that nobody can explain." Nice. And, of course, there are the god candidates themselves, who all have strange powers that exist outside the class system. It broke my heart when Factol Ambar referred to them as "spell-like abilities" in an ostensibly in-character passage, but it's still nice that there's a guy who can control the weather without having to be a level 8 weather-mancer or something.
The best part of the adventure was probably the sub-plot that revolves around Trolan, the other person the succubus released as a distraction. He had a weird romantic obsession with the Lady of Pain, and it's a mystery why he's still alive. Apparently, instead of killing him, she had the Dabus escort him to Harbinger House, where he could be safely tucked away with all the other potential gods. The succubus impersonates the Lady of Pain and tells him that she wants to build her a cult, but of course the real Lady of Pain hates being worshiped and a bunch of cultists die as a result. Trolan emerges from this plot none the worse for wear, and that's pretty weird.
I feel like an entire adventure could have been built around trying to discover this guy's whole deal. Maybe learn some new info about the Lady of Pain and the inner workings of Sigil along the way.
The last thing I need to talk about is something that's been a long time coming. Planescape, as a whole, is really weird about mental illness, and it kind of bums me out. Some people are just "barmies," and that's that. You can tell when someone is barmy just by looking at them, and there's a certain interchangeableness to the characters that indicate they share a common affliction - they all act pointlessly wacky, and some of them are unpredictably violent. It's just a bunch of gross, ableist tropes and I guess that's just what the 90s were like. I've made my peace with it, but it was especially dense in this book, and it didn't need to be. The guy who can control the weather with his mind is clearly operating on a different level than the rest of us, and it would have been a lot more interesting to treat his viewpoint with respect than it was to say he "worships an unnamed god of evil who he refers to as 'the Mad One.' The barmy priest believes that the best way to convert a sod is to bash his head in."
Overall, I'd say this is entirely skippable. There are worthwhile elements, but the whole thing is a big shaggy dog story. Even if you totally whiff it and create a new god inside of Sigil, the Lady of Pain shows up at the last moment to banish him. The villains' plans are nothing compared to the overwhelming power of canon.
Ukss Contribution: Despite my disinterest in the plot, I'm actually digging many of the adventure's fantastic conceits. There's a universal gate key in the possession of Factol Ambar, and it kind of hit the right balance between spooky (it's made of an unknown blue stone and it's constantly changing shape and sprouting spiky protrusions) and greed-worthy (with it, you can go anywhere!). But then the book decides to give it the inexplicable secondary power of constantly recording images of its surroundings, and it becomes entirely too obvious a plot coupon.
I also really like the idea of a boarding school for would-be deities, but I feel like that's an entire campaign pitch on its own (either that or the premise of a series of YA novels).
So my choice here is Galkin, the guy who can turn himself into a lightning bolt. This isn't super useful to him, except when he wants to clear out some meddlesome PCs, but I like non-biological shapeshifting, and I love that this guy doesn't even transform into normal matter. Maybe I'll make a whole species of lightning people.