A phantasmal hand grabs a tentacle, damaging it for 6. The waterfall's roar covers almost all sounds. Suddenly Derendil drops in the water. He takes a moment to orient himself and starts swimming towards the monster. Eldeth manages to get in range and swings the shortsword that she just got from the armory (to hit: natural 1), but only splashes the water around her.

Initiative order:
22 Arokas
20 Ront
19 Derendil (36)
18 Sarith
15 Eldeth
14 Tanis (holding Stool)
14 monster (6)
11 Thunk
10 Drogen

Tanis goes next, then the monster.

Tanis carries Stool as far as he can west towards the shore (30'), then sets him down to float on the water (movement and free action).

He then rapidly fires two bolts with the drow hand crossbow at the tentacled monster, using his long practiced expertise with such a weapon.  He concentrates on his instincts as a hunter (Favored Foe, adding 1d4 damage once per turn on a hit).

Action:
Hand crossbow to hit:  10 (assuming this hits since Arokas' Chill Touch hit with the same roll).
Hand crossbow damage:  8 + 3 from Favored Foe = 11

Bonus Action:  (Crossbow Expert feat bonus attack)
Hand crossbow to hit:  27 (Natural 20)
Hand crossbow damage:  9 (add 3 from Favored Foe here if first attack missed, plus 4 more from crit, so +7 extra if applied here for 16).

(Subtracted 2 more crossbow bolts from ammunition, but Tanis now has several cases worth of bolts).

Two bolts hit the tentacled monster's central mass. It flails and thrashes wildly for a few seconds and suddenly stops moving. Its body floats to the surface and you identify it as a gray ooze.

Eldeth and Derendil swim and wade out of the pool (I'm guessing everyone else is moving out of the water too, but I don't want to assume). Stool keeps very close to Tanis.

As you get out of the water, you hear shouting from above. A shower of blood falls from the bridge, closely followed by Jimjar, Buppido, Topsy, Turvy, and Sarith. (common) "They killed Ront and Shuushar!" shouts Jimjar while sprinting to the west tunnel "time to live another day!"

((Hmm, Think in water.  Can Thunk swim?  Say 50/50 - Evens he can.  13 ))

…

     Thunk splashes.  Thunk flounders.  Thunk gasps for air and coughs out water.  Thunk struggles loudly, Thunk doesn’t want to drown!  Thunk tries to move towards land, Thunk just cannot figure out how.

     Thunk goes under once…

     Thunk goes under twice…

     Thunk goes under the water a third time and doesn’t re-emerge.

     Thunk doesn’t re-emerge until he stops floundering and just stands up.  Seems by the time Thunk really panicked he was in the shallows anyway.  Thunk laughs and runs to join the others…

Arokas rushers over to the tunnel and helps usher everyone through. Once everyone is through, he spreads his Caltrops (bag of 20) across the ground in front of the tunnel so anyone chasing after them will step on the caltrops. After emptying the bag, Arokas casts Minor Illusion to make the caltrops disappear, disguising them among the ground (This might case disadvantage on the pursuer’s saving throw since they don’t know the trap is set, but it’s up to the DM). He rushes after the group once he is finished.

After the monster is dead, Drogen follows the others into the tunnel. 

Tanis does not attempt to recover his bolts shot into the ooze.  Making sure everyone still alive makes it out of the water (and glancing in disbelief at Thunk for a moment), the ranger picks Stool back up and heads for the western tunnel.

"I am saddened to hear the drow already killed two of us.  Especially Shuushar."  (Ront, Tanis could take or leave).  "We must move fast and gain a lead on any pursuers.  Everyone be on the lookout for water or anything edible as well, since we have none."

You hear crossbow bolts hitting the cavern floor as you make your way to the western exit. The drow are not eager to jump into the pool below, so you can still put some distance between your group and them if you keep going nonstop.

Sarith moves to the front of the group and points to a tunnel in the subterranean maze, taking advantage of the telepathic communication that Stool opened for everyone "most patrols follow that path" then to a smaller passage "that one is rarely used, but it would take longer to get to Neverlight Grove." Stool, still clinging to Tanis, agrees "yes, Neverlight Grove." Buppido scoffs at the suggestion "Gracklstugh is were we should go, the duergar city is strong and has everything we need to."

Arokas catches up to the group as Sarith starts speaking. “If it’s longer to Neverlight Grove, then we might want to go to Gracklstugh. We might be able to loose our pursuers in the city, as well as stock up on supplies before we go to Neverlight,” Arokas telepathically says while he pants. “That’s my thoughts, but I will go wherever the group goes.”

     “Duergar nice?” Thunk asks. “Duetgar not lock us up?”  Think stays near the front but cannot lead for he has no head for directions.  Shocking, isn’t it?

"No, duergar not nice, Thunk," Tanis growls telepathically, remembering how he had gotten trapped in the Underdark to begin with.  "They are devious and evil, enslaving or killing people of my homeland.  I know the surface dwarves of Icewind Dale, and I number many among them my friends.  But the duergar, the grey dwarves - they are different.  We will find no friendship there, only treachery.  We should go ANYWHERE except this Gracklstugh.  And I did not hear anyone say Gracklstugh is closer than Neverlight Grove.  Only that Buppido wants to go to the duergar city.  Perhaps we make for this darklake first, then decide?  It may be easier to lose the drow pursuit if we can find a boat and travel over water so that we do not leave any tracks."

Tanis looks to the dwarves (Drogen and Eldeth) for support here.  Surely, they must know of the duergar's evil ways.

To Buppido, Tanis says coldly, remembering the derro sneering when he expected one of the prisoners to die in a fight.  "You are free to make for Gracklstugh.  I'll not stop you.  But nor will I follow.  We should take one of Sarith's passageways.  One sounds shorter but with more drow patrols, which would make me lean towards the longer one.  Yet I wonder what other dangers the Underdark may hold there that the drow keep clear on their route."

Sarith takes some time to recall what he knows about the underground routes "the Darklake's closest point is roughly 8 days march west from here. There is a kuo-toa city near its eastern end. Gracklstugh is further away, almost at the south west end of it. And Neverlight Grove is almost a week past the western end of the Darklake itself." He stops for a moment and continues "the Darklake is huge, a complex network of rivers, tunnels and canals connecting multiple water filled caverns. It may take 20 days to cross it in its entirety."

Buppido does not try to hide his comtempt "the fungi people are weak, the duergar are strong."

Jimjar suggests going to Blingdenstone, a deep gnome settlement. "There is a passage going up to the surface world they use for commerce, but getting there means going across or around the Darklake, my home city is nearly 30 miles north of the Darklake."

OOC: whatever way you decide to go, you will need to decide the marching order you will use. Some tunnels are narrow and will force you to go in single file, in other you could go two abreast. Large caverns also exist.

OOC2: please roll 2 perception checks to speed up things in the future.

"Aye the duergar are no better than our drow captors. I have no interest to cross paths with them or seek their assistance.  We have no water or rations; it may be wise to travel to the closest destination."

Tanis listens to Sarith, hoping that it is safe to trust the drow who had been imprisoned with them, then eyes Drogen in appreciation of the dwarven cleric's confirmation of the nature of duergar.  Many options.  The Kuo-Toa city by Darklake, though without Shuushar, we may not be welcome there.  Neverlight Grove for Stool.  Blingdenstone - we have three deep gnomes with us and they may wish to head there.  Hopefully not Gracklstugh.

"It sounds as if we need to head for Darklake first as it is the closest and a waypoint to all of these other places.  And again, if we can find a boat to travel by water, it may obscure our trail for any drow pursuers.  As Drogen says, we have no water or food.  Let us take the shortest path there."

As a scout, by training at least, Tanis volunteers to take the lead (first in marching order), straining his eyes into the darkness with darkvision.

Perception roll #1:  25
Perception roll #2:  26 (natural 20)
Stealth if necessary:  19

The dwarf will take up the rear once they’ve decided what direction to take.

perception 1: 11

perception 2: 20

Arokas shrugs. “Alright, guess we’re headed to Neverlight,” he says. Arokas gets near the back, right in front of Drogen.

Perception 1: 6

Perception 2: 13

     Thunk shrugs and follows whoever leads.  All these words and names and such make his mind wander and he's not the sharpest tack when he is paying attention.

Perception:  Nat 20+0 for a 20 and a 12+0 for a 12...

Seeing that most, if not everyone, in the group are aginst him, Buppido keeps walking quietly while picking at his fingernails with the point of a drow dagger.

The tunnel grows darker as you put more distance between your group and the drow at Velynvelve. Less than an hour in you are all relying only on your darkvision to find your way around. Sarith keeps scratching his head as he walks next to Tanis, the drow recognizes the way easily and nods to himself when passing weirdly shaped stalagmites.

About an hour after your escape you suddenly stop hearing everyone else's toughts on your mind. Stool's rappports spores effect has ended. The short Myconid follows Tanis as quickly as it can, barely paying attention to anything else to keep up with the taller races.

Two hours in the ground is widely uneven. You notice a few passages left and right that Sarith ignores. Topsy tries to go into one of them on his own, but his sister stops him before he splits from the group. Most caves are silent, but from time to time you hear the fluttering of wings and slithering of bodies that make you realize that you are not alone in the dark. Other creatures lurk nearby, waiting for prey to come their way.

You keep pushing forward, the silence becoming oppressive, only to be broken when Derendil catches up to Tanis and Sarith at the front of the group (elvish) "glad I am that we are no longer in the cell waiting to be killed or sold as slaves. But my body can scarcely go on any longer without food or rest."

OOC: congratulations on escaping from prison! You all go up a level.

Arokas collapses on the ground, panting. “Please…I need…a break…too…” he says. Arokas gets back up, legs shaking. “I can’t continue…at this pace until…we get some rest.”