I was torn between giving this one 2 stars because of one huge glaring problem and 4 stars because it delivers well on the central premise of the AP, so I've settled for 3 stars as a compromise. Let's get the huge glaring problem out of the way first: this book has *a lot* of combat encounters for which no battlemaps are provided. This is, frankly, inexcusable in something that purports to be a ready-to-run adventure. There are nine potential combat encounters described in the first chapter; a map is provided for only one of them. Pathfinder 2 is a very combat-focused system and the AP is written with plenty of combat encounters; surely you can ditch some of the extraneous background lore to create space for things that are essential for those combat encounters to be run!
With that said, the good stuff: you get a fun trek through different parts of the Darklands with lots of good RP opportunities, visiting a twisted fey settlement, a couple of neat side dungeons and travel encounters, and a hryngar/duergar city. The BBEG is introduced, lots of stuff happens, and it's all nicely connected to the overarching plot and themes of the AP.
The premise of this book is great but sadly the execution leaves a lot to be desired. The adventure opens with a sort of urban sandbox chapter, which is a nice idea but has several major problems. First, the book does not include any maps of the city; to get those, you have to buy another book altogether (Lost Omens: Highhelm). Second, the locations within the sandbox are almost completely disconnected from one-another and from the main plot of the AP. As a result, the encounters and challenges within each one feel like pointless sidequests and there is no logical chain of clues or leads that the PCs can follow from one location to another; instead the adventure just calls for them to seek out rumors. Third, the PCs' motivation for engaging in this filler content is incredibly flimsy - rather than having any sort of concrete objective, their patron NPC simply tells them to "go out and find trouble". Fourth, many of the encounters at the sandbox locations are resolved using subsystems, which is fine, but those subsystems are presented in an excessively verbose manner; the author frequently waffles on for 5 or 6 pages without providing any kind of short summary of the encounter's structure that can be referenced while actually running the things. The pacing is also very uneven; chapter 1 features half a dozen major locations statted for level 1 PCs, which my group completed over the course of around 14 hours of session time, then a single mini-dungeon with 2 encounters and 1 hazard that is statted for level 2 PCs, which occupied about 3 hours. Upon completing that mini-dungeon, the PCs are expected to be at level 3!
I found this book extremely disappointing, especially after having run Night of the Grey Death, which has an excellent urban sandbox opening chapter. Unfortunately, I will probably be making a point of avoiding any APs featuring books written by the author in future.
