Well, the final encounter took 3 hours. We started Session 3 with the dragonship beached at the ziggurat, and I told the players that they ought to take stock of where they've been, what they've got, and what clues they've seen.
Then I turned over the 3 minute sand timer.

They decided to try the censer with the incense, and Juan Delgado (played by Benjamen), the chaotic warrior with the black silk tabard and chaos sigil (of Shub-Niggurath, in my campaign) went first. The beastmen genuflected to him as he drew near and they noticed him, and growled in homage, "Shub! Shub!" Several other characters followed behind carefully (Leeroy Jenkins, Lawful Thief; Yngwie Malmsteen, Lawful Dwarf - both played by Carsten; and nameless wizard's apprentice, 0 level but very chaotic, played by Zack), wearing the wormy robes from the pool room. The rest of the characters hung back, unnoticed by the beastmen, who were busy with the ritual and the sacrificing of the villagers.
The four advanced slowly, soon attracting a retinue of beastmen groupies all chanting "Shub! Shub!" Occasionally, across the ziggurat, there could be heard an echoing chant of "Ni-Gu-RATH! Ni-Gu-RATH!"
This was when things got deadly.
Dolores, the chaotic Thief, snuck up behind the slowly advancing group with the beastmen groupies and backstabbed the one in the rear. Nobody noticed her silently killing the trailing beastman, and so the group kept on advancing around the ziggurat, and Dolores kept up slaughtering the beastmen. It was pretty epic, she took out about 5 or so beastmen before she missed.
Emboldened by her success, a halfling (played by Benjamen) ran up and tried to attack some beastmen, but missed. They grabbed him, and hauled him up the ziggurat.
Then Shun Tai the elf (played by Zack) cast sleep on the beastman shaman and an acolyte, and succeeded. Since he was close to the magma, Shun Tai wondered aloud "will he fall into the magma?" I gave him a reflex save and he failed, so into the magma he went, with the effigy. Boom. The party cheered.
About this time all hell broke loose and Juan Delgado charged up to the top and started meleeing beastmen. The halfling who'd been nabbed burned a ton of luck to get loose just before they tossed him into the magma, then he fled back to where he'd dropped his mithril shortswords (more on those in a bit, heh). Realizing the ruse, the beastmen engaged the party and a pitched battle ensued.
The party took out a few more beastmen with no losses, then Molan appeared out of the magma and everybody went "ruh roh." Shun Tai, however, used sleep AGAIN to great effect... there's nothing in the module that says he's immune to sleep, so... failing the will save, Molan went down, along with another acolyte. Juan Delgado tried to take out the sleeping Molan, but didn't do enough damage and he woke up.
But I don't think he EVER hit anyone with the flaming flail. It was very disappointing. Juan Delgado disarmed him with a mighty deed, which was cool. Then the 0-level wizard's apprentice sprinted to the top and grabbed the flail. Next round, however, because his luck was so low at that point, Molan turned to strike him ... and critted. The crit roll was to disarm the target's weapon d20+5' in a random direction, so the flaming flail went flying and, of course, the 0-level wizard's apprentice was dead.
It took them a while after that to mop up the beastmen, and Fatty McGee went down twice, being healed once by John Craven the cleric, but then John Craven failed his heal roll the second time (incurring MORE anger from Crazy Old Testament God / Jewish God, I think he's on a 6 or less now). Shun Tai and another elf, Wei, both went down and weren't healed, and a couple of 0s bought the farm as well... but ultimately the party prevailed. Bodies were rolled over and Shun Tai and Fatty McGee lived, at the cost of MORE stamina loss (they were the ones who went down in the tower above).
When Molan disintegrated into the magma pillar, the halfling with the mithral shortswords ran up and attacked, easily sticking the magma pillar with the swords, and promptly melting them. No idea what the melting point of mithral is but I'mma guess magma ought to have some meltacious effect on it. Heh.
Then the ground started shaking and a few characters did some quick, brief looting (Juan Delgado grabbed the blacked plate mail) and they got back onto the boat before it was swept out through the collapsing cave. We ended there as it was late.
Lessons from this session:
Sleeping targets are attacked at +1d, not just +2 or +4. So, good to know.
Can you crit on a 20+ if you're attacking with a d24? Sure seems like you should.
Deity disapproval... uuuuuuugh... clerics are really tough. Or, this guy's luck is just seriously awful.
Sneaking/hiding/backstabbing is powerful, daggers rock. Thieves are pretty badass.
Next up: a choice for the party... left(south, following a small stream) is Hirot and Doom of the Savage Kings, right(north into a canyon/badlands) is People of the Pit. We'll see what they choose!