The party begins to slog their way through the caverns and chambers of this interminable network of tunnels and natural features, keeping their way by following the stone path that was smoothed by the trod of untold numbers of victims who made their way. The stream turns away from your path, and in succession you climb and fall, climb and fall, losing all sense of direction, depth and time. Eventually you learn to stop when you're tired, having little else to tell your senses that rest is needed.
Over time, you've begun to straggle on the journey. This can't be helped. The darkness means that often you can't see your companions, unless you shine a light on them; you're walking through a dream-like mix of shadow and yellow glow. Often, you're following someone ahead who is naught but a silhouette in the lantern their carrying ... or you've fallen behind as you try to light a torch, or find something in a bag to nibble upon, or relieve yourselves.
If not for the path, the caverns would be treacherous. There are pitfalls and razored stone that would tear the soles of your boots to ribbons. There are rock falls that might collapse under your weight. There are places where sheets of water drift down from the ceiling, ice cold, only to disappear in cracks and crevices in the rocks ... but the path avoids these hazards.
You're not excessively straggled out at the moment you come around a corner and discover yourselves face-to-face with a caterpillar. Choosing two random people who happen to be at the front of the column at this moment, I find Engelhart and Rob. Willa is behind, then Mikael, Lothar and Embla pulling the rear.
That's when you encounter this:
There's no need to roll surprise. It is apparently just sitting there. It looks like a cross between a centipede and a caterpillar, with hundreds of fuzzy spikes running along its back (not exactly like the picture, we do the best we can).
After the party takes two rounds of movement, updating the image: