With the cultists and acolytes all slain, and Brother Sampson captured, they proceeded to interrogate him, which was by turns useful, infuriating, and creepy as hell. They learned that the cult within the caves was known as the Order of the Mask and Tattered Shroud, who were dedicated to a god(?) known by turns as the Yellow King, the Veiled King, the King in Yellow, or Hastur. (There was some ambivalence about this last part. Hastur was what was behind the Yellow King's mask... maybe? Brother Sampson's ramblings were hard to follow.) The gist of it seemed to be that there was a high priestess, The Yellow Lady, who had (or claimed to have) some kind of claim to the throne, and was raising an army to go get it, at which point the Yellow King would come from his city of Carcosa (some place where the sky was yellow and the stars were black) and marry her and they would rule together.
(And by "raising an army" he meant reanimating the corpses of all the orcs, goblins, and other humanoids wiping each other out in the Caves of Chaos.)
Brother Sampson also gave them some intel about the general layout and power structure of the caves, informing them that the gnolls were the group currently most favored by the cult.
Once they got all they could out of him for the time being, they tied him to a tree outside before heading back into the temple to scout around a bit more. While they were outside, they spotted something that rather took them by surprise: a band of kobolds, maybe 30 in number, streaming out of the cave they had raided the day before. They were carrying bundles and marching– the surviving kobolds were fleeing the Caves of Chaos. "An apartment just became available!" quipped Brother Drang.
The party cautiously made their way back into the temple; Togar used his divine sense and quickly came to the conclusion that there undead in the various other chambers all around them. They also found what appeared to be some kind of dark altar that radiated strong evil, although not exactly diabolist or infernal in nature, so much as "the universe is sick here." Somewhat baffled, and not eager to take on "an army of undead," the characters retreated from the temple and decided to head for the gnoll cave instead in the hopes of finding and freeing Lady Cynthia.
They did not get far. A bad Stealth check alerted the gnoll guards at the entrance of the party's presence; Nikki, dressed in robes purloined from the dead cultists, said he'd come to check up on the lady they'd taken prisoner. This seemed to baffle the gnolls, and when they turned away to confer with each other, the group swarmed in and attacked. Two of the gnolls fled for reinforcements, and this led to a chase further into the cave.  There was a pitched battle in the corridors, during which both Sheala the wizard and Rina the wood elf ranger got knocked unconscious, but a natural 20 on a death save and a healing spell brought them back up respectively.
When the guards and their reinforcements were defeated, the characters retreated, blocking the corridor with burning oil to forestall pursuit. They came out to the ravine to find Brother Sampson, still gagged and tied to a tree, snickering at them not unlike Tim the Magician when Arthur and his knights were forced to run from the killer rabbit. They decided they'd had enough of him and marched him back to the Keep. On the way, they spotted the marching kobolds setting up a camp down in the river valley, and mused briefly on the difficulties that lay ahead for the tiny saurians. "Not our problem!" said Nikki.
Back at the Keep, Bailiff Delahue took a keen interest in the emblem of the Yellow Sign they'd taken from one of the dead cultists, and told them to show it to Captain Helgist while she clapped Brother Sampson in irons (and left him gagged, as he was a spellcaster). Captain Helgist, in turn, informed them that the gnolls who'd captured Lady Cynthia were wearing emblems like this as well, and went to report the party's actions to Lord Blakewell, the Castellan, and told the party that they should come back later for further instructions.
Content to the let the caves stew in their own gravy for a bit, the party then headed off to a theoretically-abandoned watchtower to the south of the Keep, where they'd spotted plumes of smoke rising from fires the day before. Some reconnaissance revealed the watchtower to be the lair of Red Hand Harry and his gang, the highwaymen who'd been raiding caravans between the Keep and civilization. The party waited until the wee hours of the night, when most of the bandits were asleep except for a couple of bored guards, and struck!
The guards were taken out quickly and quietly; Nikki then used his thiefly skills to block the doors and spread oil at the top of the stairs in the tower, and they began their assault. Brother Drang cleared out the entire bottom level of the tower with a thunderwave– announcing their presence in a dramatic fashion. The bandits, all rudely roused from their slumber, grabbed up their weapons but had no time to don their armor. What followed was a wild and chaotic fight, with some of the bandits fleeing, some of the bandits fighting back, and some of them slipping and falling on the stairs.
Red Hand Harry himself joined in the fight until Miskan warped his mind with dissonant whispers, causing him to flee. That almost backfired, as the reward the party was chasing was only for Red Hand Harry himself, and if he'd gotten away it would have been 500 gp lost to the night. Miskan gave chase and was able to follow up with a sleep spell, and Harry was out like a light.
Sheala, meanwhile, had gotten herself into a 1-v-1 with one of the bandit archers, who were much more capable than most of the bandit rabble. He was trading arrows for each of her rays of frost, and she ran out of hit points before he did. For the second time in as many days, she fell unconscious, this time bleeding from multiple wounds. Fortunately, the rest of the battle had been more or less wrapped up by then, enabling Togar and Brother Drang to restore the fallen mage.
Sorting through the items in the tower revealed that this gang was quite definitely responsible for the disappearance of the caravan that Curian the Jeweler was so desperately waiting on– and that the caravan was likely to never come now, given that everyone in it had been sold to the Lady in Yellow as slaves. They were able to retrieve a variety of trade goods, however, including several bottles of Appletop Wine, made with the rare honey from a colony of giant bees . Nikki claimed a bottle or two as "carrying charges," and the party decided to camp in the outbuilding for the rest of the night, tying up their prisoners and leaving the piles of bandit bodies in the tower.
 I actually misread my adventure key in this part, putting the gnoll commons in what was supposed to be a storeroom. Oops. The fight would have come out much the same, I suspect, except the room beyond was not intended to be full of gnolls. Oh well, retroactive revision is a thing! ;) This is something that occasionally trips me up in the old-style "every room is a 30' by 30' square" style dungeons... with no clear way to distinguish one room from another on the map, I sometimes get lost in the room numbers. But it's kinda like the Quantum Ogre... the dungeon doesn't "actually exist" until it's encountered by the players!
 Wibbly-Wobbly Continuity-Wontinuity. This is actually a reference to "Buzz In the Bridge," an adventure I ran with my 3.5 group something like ten years ago, back when Ryan was in the group instead of Sirfox. Teeeechnically, this game takes place earlier in the world's history than that game, so Appletop Wines shouldn't be a thing yet. But really it's just a game, I should really just relax.