Yeah, I screwed up. It turns out that in my recollection of events, I had Sigbert with the group that went to the Isle of Mists, but he was actually with Gareth and Esma. So there are three alive in the depths of the spire, not two. Apologies for the recalibration. Luckily, Sigbert hadn't done much of note up until this point. But he is about to get his moment in the spotlight (or utter darkness, as the case may be).
Gareth and Esma quickly set about starting a fire in the kiln for light (and heat, since everyone is freezing from wading in the underground stream). Sigbert cautiously moves down the southwest passage as far as the ambient light will allow, and reaches an open door, beyond which it is too dark to see. He hears the barking and yipping of thornlings in the near distance, and a deeper growling mixed in with their voices. The sounds grow louder. They're coming back, with reinforcements!
Sigbert calls out to Esma for iron spikes, and she runs to his position. He slams the door shut as the howls of the thornlings ring out clearly just ahead in the darkness, grabs a piece of loose stone off the floor, and begins to hammer the spikes between the door and its stone jamb. He gets two spikes in before something hits the door from the other side. The door holds. Sigbert and Esma hear the deep growling again, look at each other, and run.
There's a big thump on the door, louder than before. It's the bugbear that killed Osric, but they don't know that. I roll the bugbear's Fortitude modifier against DC 15 for the spiked door. The door continues to hold.
They have a hurried exchange with Gareth in the kiln room (now quite warm and glowing with orange light from the fire), and decide to run back to the underground stream. Gareth grabs a burning brand from the fireplace to use as a temporary torch, Esma gathers up as many of the clay figurines as she can carry — who knows, they may have some significance — and Sigbert grabs the buckler off the dead thornling. As they flee back up the passage toward the totem cavern, they hear the door splinter and break (the bugbear made his next Fortitude check).
Frantic, they search everywhere, pushing and feeling the slime-coated wall. Sigbert takes the brand and holds it high over his head, looking up in an effort to see where the water is coming from. About 20' up, he sees a horizontal slot in the wall, about 2.5' wide and 1.5' tall, from which the icy water issues. He hands the brand back to Sigbert, unlimbers his coil of silk rope, and ties his knife onto one end. They hear the echo of the thornlings spilling into the totem cave, then shrieks and howls as the body of their revered witch doctor is discovered.
Sigbert whirls the knife on the end of the rope and throws it upward at the slot. It's too dark to really make a clear throw, so I rule he needs to make a Luck check. He rolls a 2, and his Luck is 9. Incredibly, the knife lands inside the slot at one side, and the rushing water wedges it into a corner. Sigbert give sit a tug and it feels secure.
Gareth, realizing their pursuers can see the light of their brand in the otherwise dark cave, takes the rope in one hand and drops the burning piece of wood into the water, where it sizzles out. Utter darkness engulfs them.
Gareth, the lightest of the three at 4 and a half feet tall, scrambles up the rope. The vertical face is slick with slime, and the sheet of water complicates things, so I give him a DC 12 Agility check. He rolls a 15, and scrambles up to the slot, showering water on Sigbert and Esma with each slippery step.
Sigbert and Esma hear thornlings splashing into the stream, barking and growling as they make their way past the dragon totem and toward their position. The dog-men must have seen the light of the brand before it was extinguished. Sigbert braces himself, readying his newfound buckler, and Esma starts to whirl Pierce's net in front of her, hoping for some defensive benefit. They stare into blackness, listening for the splashing footsteps of their hunters.
Gareth reaches the slot and climbs through, and, trying to sort out his physical surroundings by touch, finds himself in a stone channel through which the water flows at a depth of about 1". He rolls around, braces his feet on either side of the slot, grabs the rope, and tugs on it as a sign to Sigbert.
There is a loud bark as the thornlings sight Sigbert and Esma (darkvision). Two primitive wooden spears come whistling through the darkness. One strikes the stone wall over Sigbert's shoulder, and the other is deflected by Esma's spinning net (a straight up miss, narrated for color, although I had given her a temporary +1 AC bonus for creativity). Both of these ranged attacks were made at a +2 bonus for Sig and Es being effectively blind.
Sigbert hands the rope to Esma and shouts "Go!" She drops the net and ascends as quickly as she can, making the DC 12 check. Leaving Sigbert alone against unseen enemies.
He hears the splashing as the two nearest thornlings rush at him. Having thrown their spears, they only have claws to attack with, but he doesn't know this. He sidesteps as one attack misses, and on the next attack I roll a 20. I lift my judge's screen to show him the roll, and tell him that he can sacrifice his buckler to negate the hit before I roll the result. Having 2 hit points left, he chooses to do so. I rule that the thornling's claws rake into the buckler and tear it from his arm.
He tugs on the rope and Gareth and Esma pull with all their might, lifting him free of the fray. One of the creatures gets a free attack because Sigbert is disengaging, and misses. Between climbing as best he can and being pulled, Sigbert makes it to the slot and through.
The three flop out of the water channel onto a cold stone floor, soaked to the bone and shivering. The chamber echoes with their gasps, along with the the sound of flowing water and the distant howls of frustration.
All is black.