PART FOUR: THE RISE OF YDERSIUS
This part starts off by dictating that there is only one strategy that is likely to succeed, because if the PCs don't follow it, you're to swarm them with all kinds of high level monsters and NPCs that can easily TPK the party. They are to use the army as a distraction to keep defenders busy, while they sneak into the inner sanctum to interrupt the Ydersius-raising ritual. This is, of course, a dungeon with traps and wards and loaded with monsters.
There's also some odd hazards like collapsed tunnels of magically treated stone that cause spells to go awry and even cause spellcasters to take a bunch of non-lethal damage. You start off fighting starving morlock slaves, a snakey hydra (more snakey even than normal), snakemen temple clerics and snake-shaped stone golems.
There's actually a system in place to determine how successful the PCs were in creating a distraction army, and depending on how well they did, you can have more or less individuals here to fight; if you didn't do well with your army, your foray into this ruined temple will be harder.
Then you start hitting actual characters. Nazith-Yol is a restored ancient snake-man who likes to build weird snake-clockwork golems and stuff. He's a tough fight. Then, we have Khorak, an Ydersious worshipping troglodyte fighter, with temple guard companions or sidekicks. Nylla-Jas is the "wife" of Nazith-Yol, and you might end up encountering them together, especially if the latter flees to the former after being beaten by the PCs earlier.
As you get further into the sanctum, you fight an avatar of Ydersius called the Emperor of Scales—a gigantic snake-head with a bunch of snake body/tails coming out of it like dreadlocks, or something. And iron golems. There are more magical traps, and a marilith, which actually makes some sense, given the snake-like aspect of mariliths.
There is a Chamber of Gates, but the Gates are broken. I'm not sure what the purpose of this is except maybe to give the PCs something to do when they finish the adventure; repair them and go off exploring the exorealms beyond? There's a chamber with animated shed snake-skins, and another snake-man champion, Khestath, who's a fighter/ranger. As always, snake-men priests are with him.
And finally, you reach the innermost sanctum of sanctums, fight the BBEG Vyr-Azul and his giant albino snake Qestl. It turns out that they haven't interrupted the ritual, however. It's already happened. Ydersius has started to regenerate, but the PCs arrived earlier enough that they only need to fight a weakened avatar of him, not the god himself. (Itself?) It crawls up out of a chasm in the floor that goes deep into the Underdark (sorry; Darklands) after the high priest is defeated.
There's a few more rooms detailed after this, but it's the end.
CONCLUDING THE ADVENTURE
To really have "won" the PCs need to sever the avatar's head again, otherwise, it'll grow back in a few days, or some point in time, at least. Killing Ydersius for real sends "psychic shockwaves" to all of the snake-men in the world, though, making the battleground between your army and the snakes a handwavey deus ex machine win.
It's suggested that Savinth-Yhi will soon face a wave of settlers and colonists, becoming another major metropolis rising from the ruins. The snake-men will plot their revenge in one way or another. But what if the PCs don't actually defeat Ydersius' avatar? In this case, his restoration will eventually be complete, but he'll be right here among his people, who will face a totally different kind of "psychic shockwave" and the serpentmen will soon become a major threat to the surface world.
BEYOND THE SERPENT'S SKULL
There are a number of possible adventure seeds spelled out that are obvious follow-ups to the adventure as it proceeded:
- FACTION REVENGE: The PCs had a rival faction, which they almost certainly had to fight, and probably eliminate their expedition. The parent faction, however, lurks still seeking revenge for the PCs having foiled their plans. It's entirely possible, of course, that more than one such faction is unhappy with the PCs following the expedition to Saventh-Yhi.
- THE FATE OF SAVENTH-YHI: as spelled out above, how with the PCs respond to the arrival of "claim jumpers" and others; or will they attempt to keep the city's location a secret?
- SAVITH'S CRYPT: A big dungeon next to the city. Sigh.
- FINISHING SAVITH'S JOB: Can the skull be destroyed? Dunno, but the PCs can maybe find out, which would put Ydersius into the "dead gods" category permanently. The skull itself is a powerful, insidious and dangerous artifact.
- RISE OF THE GORILLA GOD: The Gorilla King from earlier on might become a "nascent demon lord"—on his way to becoming a true demon lord. He's actually statted as a CR 23 unique demon, and would be a potential rival to the cult of Angazhan, which would cause all kinds of weird demon-monkey political games, if the PCs were interested in political infighting between demon-monkeys.
- THE SERPENTWAR: Even Ydersius' defeat probably "awakens" the slumbering, or at least peaceful and quiet snake-men populations underground, causing all kinds of trouble. Armies of snake-men troops and their enslaved Darklands auxiliaries invade all over Golarion. Thanks a lot, PCs! There's even a CR 20 snake-man general statted up, Sargogen, Lord of Coils, and a few stats for a mass combat minigame (buy another Paizo product to get it! Sigh.)
Following this, there's a section describing Ydersius and his cult among the serpentmen, part 6 of 6 of a short-story that's run throughout the adventure path, and, of course, the little Bestiary. Which contains:
- Wandering monster tables specific for this adventure
- BAREGARA—a bigger, meaner demonic ape than the bar-lgura. Which, curiously enough, Pathfinder doesn't have. I guess they were never open content.
- EMPEROR OF SCALES—as described above. Uh-gly.
- GROOTSLANG—an elephant-snake, and supposedly a cryptid of South Africa (hence its Dutch name.)
- HOLLOW SERPENT—in greater and lesser varieties; the animated shed skins of giant snakes.
- KONGAMATO—some pterodactyilish creature from Zambian folklore or somesuch. Paizo can't resist showing off that they did their research in African native folklore.