Shortly after a raised rock interrupts the water on the floor, the cave branches to your right. A careful diversion there and the slaying of a smaller frostvenom spider will yield you some blisterwort and bonemeal for alchemical use. Return to the main line of the cave and continue carefully advancing deeper.
You'll soon find an entrance to a much larger cave. The water ends here, and you can see a smaller frostvenom spider wandering around- don't charge in recklessly, though! Three full-size frostvenom spiders lurk here, two of them hanging from the ceiling right by the water's edge, and a third over a raised area off to the left. As full-sized spiders, these are easily more than a match each for characters under level 10, and can quickly be fatal to anyone less than about 25th level who runs in and starts stabbing about blindly. The main threat here is their ability to launch their venom, which not only eats through your stamina, but its damage ignores any armor you're wearing and it distorts your vision, making it difficult to function in combat.
On the upside, they're all such large spiders that they can only come at you down the hall single-file, and anyone with sneaking skills can likely drop them in two hits with a bow, so acting with a little caution or foresight can turn this from a deadly ambush into a slightly tense fight pretty easily.
Once the spiders are dead, you can move out to the middle of the room and check out the corpse of a second adventurer- nothing special here, but a sign that the people who came up here kept going. Look up the slope of the floor to find the lantern that sides beside the passage still further into the cave.
As you enter the passage, take a moment to gather mushrooms and make note of the fact that the dead here are burnt corpses- not intact, as would be explained by the spiders. This is your real clue that soething more is going on here.
Stay where you are and look deeper into the cave, and you can spot a smaller spider on the ground, which you'll want to quickly take out with whatever ranged offense you have. Near the spider are several mushroom patches and a head-covering on the floor- one of the elk-skull cowls that are worn by the Forsworn.
