Let's start, as always, with the adventure summary, as presented by Paizo.
After curing the half-elf Pathfinder Juliver (rescued in “The City of Seven Spears”) from her feeblemindedness, the PCs learn that she is a companion of a Pathfinder who is in dire jeopardy in a secret subterranean city below Saventh-Yhi—a lost city of the serpentfolk called Ilmurea. Retracing Juliver’s route, the PCs locate and enter an ancient vault between the two cities, only to discover the magical portal that Juliver used to reach Saventh-Yhi from the city below has been disabled and must be repaired to be of use. The PCs learn that the components for repairing the portal—six delicate focusing crystals—have been hidden in six other secret vaults beneath the city.
As the PCs begin exploring the other vaults, they become infected by insanity-inducing spores that infest all of the vaults. Meanwhile, events are unfolding in Saventh-Yhi as well. Reinforcements have arrived for one of the PCs’ rival factions, but the leader of that faction is under the domination of an intellect devourer named M’deggog from the city below. This new faction leader escalates the conflict among the factions exploring the city, culminating in an attack on the PCs’ camp. The PCs must plan their own attack on their rivals’ camp and rescue their comrades who were taken prisoner. When they finally confront the faction’s new leader, the PCs get their first clue that something sinister may be behind the conflict.
As the PCs continue their explorations, they learn that the Gorilla King Ruthazek has come down from Usaro to see this newly discovered city and is encamped outside the entrance to the final vault. The Gorilla King invites the PCs to a grand feast, where they are subjected to a series of challenges to test their fitness to rule Saventh-Yhi. Only when the PCs have passed these trials can they enter the last vault to find the final crystal they need to activate the portal to Ilmurea. In so doing, they encounter a group of bloodthirsty bandits under the control of the strange intellect devourer M’deggog, the true mastermind behind much of the havoc in the city. Once the aberration is defeated, the PCs can finally restore the portal and gain access to the subterranean serpentfolk city of Ilmurea.Sigh. Ah, yes. The multipart McGuffin quest. An old favorite, to be sure, of module designers everywhere (if not necessary so of players.) Due to some DMus ex machina, it's presumed that the PCs have not discovered the fact that each of the seven districts of the city has a "vault" or dungeon complex below it, loaded up with traps and monsters and spores. But now, we're to start exploring them! Again; I'm not really a very big fan of dungeon-crawling, underground adventures, or navigating mazes full of traps. A very small amount of that goes a very long way. It was nice in Raiders of the Lost Ark because it took maybe ten minutes (if I'm being generous) out of whole movie; which was otherwise about running around in exotic locations and killing Nazis and Moslem savages. That being said; all that really explains is my relative hesitancy to dive into this second half of the adventure path, because as I've said many times before, I'm just doing a quick summary documentation of the content, so I can strip-mine it and reorganize whatever elements of it I think are useful in a format more attuned to my own tastes. But it's harder to be motivated for a strip-mining expedition in an area where you don't expect it to be particularly abundant.
Luckily, the "vaults" are all mainly very small dungeons; no more than a few rooms and encounters each.
PART ONE: INTRIGUE IN SAVENTH-YHI Well, they can't quite get to the underground city without finding all of the pieces of the magical crystal crap that they need in order to open the vault. There are some constraints about the order in which they need to do things due to the "plot structure." This part of the adventure also details the effects of the black spores; a kind of extradimensional fungus picked up on the Lady of Death's plane many millennia ago, and now grown somewhat out of control. This seems, honestly, especially punitive—it's not a poison or disease (so immunity to such, or spells that cure such, won't help) and getting rid of the crippling paranoia that they cause is not easy. This is all well and good for an effect that is geographically limited, but in this case, it is not. The expected solution is that the PCs will buy scrolls to heal it from their faction. Sigh. Look, if you want to bleed resources off of the PCs, wouldn't it have just been better not to have given them so much to begin with?
This part of the adventure also details a timeline of events that will happen on the surface. These include the following:
- EVENT 1: THE PATHFINDER HOME COMPANION (no word on whether or not it comes with creepy groping and sexual harassment.) Juliver, the crazy person the PCs found at the end of the last adventure, is now cured and ready to talk to the PCs. This is her story. In her illustration, she looks kinda cute, but she's clearly got a crush on Eando Kline, her boss. She also spills the beans on the serpentmen underground.
- EVENT 2: A NEW BOSS IN TOWN The PC's rival faction has a new boss who arrived with new troops and took over, putting to death anyone who resisted. Two refugees stumble out of the jungle begging for aid, to be followed by 6 legionnaires, who you are meant to fight. They've also decamped from their former location and founded a new secret camp somewhere else.
- EVENT 3: AN ACT OF WAR The new rival commander is aggressive; at some point while the PCs are not in camp, he's supposed to have raided it, killed many of the camp followers (well... what I really mean is the useful members of the camp) and taken some of the PCs favorite NPCs prisoner. The dead bodies have also attracted the attention of two Dire Tigers, although they are also called smilodons.
- EVENT 4: IVO HAIGAN'S CAMP Presumably, the PCs will follow the trail back to the attackers' camp. We're told (not the PCs, the GM) that this new leader of the camp is actually under thrall of an intellect devourer; a very D&D-specific monster that looks like giant brain on four stubby little legs. Although the d20 intellect devourer didn't do mind control exactly, apparently they do in Pathfinder, since Ivo Haigan is under the mental control of this one. The camp has been heavily fortified. The camp is presented as if it's a given that the PCs will raid it. Haigan has Barsoomian white apes as guards, for some odd reason.
- EVENT 5: SCOUTS OF THE GORILLA KING For reasons that... probably have to do with being drunk and The Wizard of Oz coming on TV late at night, we've got winged, flying gorillas as scouts of the Gorilla King. This, along with additional ape activity are signs that something is up. You can fight them, I suppose.
- EVENT 6: THE GORILLA KING COMETH The Gorilla King himself comes to town to see the legendary Lost City of Saventh-Yhi with a whole troop of his ape soldiers. He invites the PCs to meet with him for dinner, and they are strenuously encouraged to attend. There's a lot of weird color (like the dinner from Temple of Doom amped up, complete with Fortitude saves to see if they get sick) and stuff that's kinda creepy, and not necessarily in a good way (naked human slaves that bow down to form benches for the PCs to sit on, for instance.) He then challenges the PCs to a series of contests, and rulership of the city is the prize. The Gorilla King himself is an interesting fella; a human trapped in the body of a Dire Ape thanks to the touch of a demon-artifact. He's actually been there for centuries, although it's often been the mind of someone else in the body. He rules over a city of awakened apes and gorillons, as well as the chaura-ka—semi-humanoid apes that are more like the chimps from the new Planet of the Apes movies, although smaller and more lithe. Not that the PCs likely care, but presuming that they win the contests, the Gorilla King only retreats temporarily anyway.
As an aside, Angazhan, the demon-lord who's vaguely gorilla like, and has actual talking, clothes-wearing, weapon-wielding apes in the jungle as his worshippers, is a cool idea. I had thought it was mostly taken from Demogorgon and repurposed to Golarion, but either James Jacobs or Erik Mona—can't remember which—told me in a forum reply that it actually started with the idea of "what if King Kong was a demon lord?" The Gorilla King isn't nearly so massive as King Kong; he's more of a Mighty Joe Young, if even that, but still—he's a pretty formidable creature even so.
PART TWO: THE FIRST VAULT I may have mentioned the keches an adventure summary or two ago; they're kind of ape-like, with chlorophyll or something like that. Anyway, the first vault is partially collapsed, and is characterized by a war or sorts happening as we speak between kech hunters and bat people. In all, spread over a few encounters, there are fourteen keches, ten bat people, an old lab for the midnight spores research with the journal of an old Azlanti priest who brought the spores back to earth in the first place. There's also a gigantic slug, a stone golem, a map of the other vault entrances, a fire-trap, and the magical portal to the city below—which they can't open until they get all of the crystal McGuffins assembled, of course.
PART THREE: THE FLOODED VAULT As the name implies, this is under the swamp lake, and is completely underwater. The spores still float around here, though. Collapsed ceilings make some of the rooms navigable (of sorts) by normal swimming, but finding the crystals (and other associated random D&D-ish treasures laying around) on the bottom might be more difficult. There are swarms of megapiranhas here, as well as a few unusual piranha-men. Seriously; the illustration gives them piranha heads (although they're not specifically described that way in the text.)
PART FOUR: THE IMPENETRABLE REDOUBT OF KHALID-SHAH Located underneath an island in the government district. There's a giant Venus fly-trap (large enough to trap humans, naturally) near the entrance. Traveling through a confined muddy tunnel, and facing paranoia effects from the midnight spores, the PCs face several "advanced ooze mephits" and a mud elemental and a muck shaitan; a type of Pathfinder genie. This is also where the healing fountain is located, as well as the skeleton of the guy who built all of these vaults, killed by one of his own traps millennia ago