Dead centipedes cover the ground, at the scene of the battle, nothing else is discovered. You do see two sets of tracks heading north, along the river.

"Up river then?" Dragen says with a frustration 

"It looks like it." Portia has stowed her blades and now has bow in hand. She steps aside and bows with a flourish "After you good sir!"

Grommok will also follow. "Where did they get those centipedes?"

“Go, now.” Gelidus immediately heads to where the tracks lead, filled with unusual eagerness.

You all take a minute to breath, after an obvious trap left in your way. It could have gone a different way if the centipedes were quicker. 

Hopefully feeling a bit more comfortable with the others around you, you step over or on the centipedes and follow the footprints.

The river runs to your right as you follow it, the footprints are not hard to follow, more obvious along the banks of the river. To your left the woods seem to get denser, thicker, light seems to disappear as you look that way.

You continue onward, the brush gets thicker, but you are able to follow the footsteps, especially the one that seem 'heavier'. As you press through some bushes, it opens, before you, a cave opening. The opening is about 5 feet high, and dark inside. You hear the slight sound of water moving beyond, there are no light sources.

[I believe you all have Darkvision except for Li. How would you like to proceed?]

Grommok will bravely venture forward, sword raised. When he reaches the entrance, he pauses, and remarks "We should light a torch for those who can not see in the dark" before continuing onwards. 

Dragen will light the torch. "We'll stick to the back and try not to alert anything within"

Portia doesn't have darkvision, so she stays back with Dragen.

“Ugh, a cave.” Gelidus would turn back but now that they’re dead set on giving this thief a well deserved punch, nothing can stop them now. “Come on! No time to waste.”

You light a torch and enter the cave. You don’t have to travel far before your stopped at the edge of an underground river, cutting through the natural stone corridor creating a 10-foot-wide chasm. The water is 12 feet below and moving fast, some tributary of the river you just left behind outside no doubt.

Across the chasm, the corridor ends at a collapsed wall. Beyond it, you can make out a room. The walls and floor of the room do not appear to be natural like the corridor, but consist of worked stone, possibly marble or a similar stone. Other than your torch and a bit of light still streaming in from the cave entrance behind you, all is dark. The sound of the fast-moving water makes it hard to hear anything else.

Directly across from you, resting against the wall is a long wooden plank, easily long enough to create a bridge across the chasm. Next to the board, there are two sets of muddy boots.

"Any ideas?" Grommok asks. "I could try swimming across and climbing up. Then I could throw a rope."

"I could maybe jump across with a running start." She looks at Grommok "or maybe you could throw me across? Either way I'll tie a rope around me, just in case I miss and need pulling back up."

"Hm. You read my mind." Gelidus looks at Portia. "That plank should make a decent bridge. Once you're across, slide it back over."

[Based on RAW: Long Jump. When you make a long jump, you cover a number of feet up to your Strength score if you move at least 10 feet on foot immediately before the jump.

Gelidus and Grommok should be able to make a running jump across. Portia would be really close and would need to roll athletics check if she wants to try. Dragen is not strong enough to make the jump.

Additionally, the river below is fast moving, probably not something you want to try to swim against. Being that it is underground, if you get carried away, you have no idea where and when you'll resurface!

The board looks sturdy enough to be used as a bridge.]

"I'll get a rope ready and we can anchor for one of you tallfolk. I know I can't make that jump."

Dragen says as he rummages in his pack for his bundle of rope

Grommok will jump across and place the board so the others can walk across. He will try to make it as sturdy as possible.

As Dragen pulls out a rope, ready to throw it across the chasm, Grommok steps back a few steps, the half-orc takes a running start and jumps across the chasm, landing without an issue.

[Grommok Perception check please as you land and grab the plank.]

Grommok takes a quick look as he grabs the plank.

Perception : 18

"Ok, I won't jump then" Portia laughs. "Once the board is in place I'll go across and check for any unexpected surprises. Don't go any further in case there's a pit."

OOC: Is there any light on the other side, or will Portia be working by torchlight?