Neth and Lia must have been past the Not-Rat when things kicked off and fled further in rather than back out to the mouth of the tunnel, presumably trying to link up with the goblin guides.

((There's a chance that Petmailer saw it before Stena was able to, with his longer sight, in which case it'd vanish before she saw it, but we'll say he let it come close enough that they were both able to get shots off before it vanished.))

((Stena's got 90' to his 120', so reasonable. How far up is it from where she's standing?))

Stena looses a shot,

Attack: 27 Damage: 15

Then, on hearing more company decides down is better and starts down the rope.

Athletics (?): 20

Stena's arrow flies true, but the creature was already half vanishing as it strikes, sending a spray of ichor, but her arrow left behind as it disappears, the shaft spinning a trail of goop as it arcs into darkness. The rope wiggles as Stena descends, Petmailer hearing a soft thud of her boots on the stone nearby.

"Careful friends, it's definitely not a rat!"
"Where is it?"

Neth and Lia's voices sound back from up the tunnel, around a corner. 

Stena: 22 (Invisible while in darkness)
Lia: 21
Not a Rat: 20 (-12 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15 <-----<<
Spiderbait:  15
Petmailer: 9 <-----<<
Galingal: 8 <------<<
Whatever is Shaking the Web: 6 (-8/??)
Jimjar: 4 

With the spider(s) disappearing into the dark, Petmailer turns and heads the other way, trailing after Galingal and heading toward the creature that is definitely not a rat.  He dashes in that direction, hand on his bow ready to stop and fire if needed.  “What is it?  What does it look like?”

Petmailer moves to join Galingal and Jimjar at the mouth of the cave, moving just a bit past Kolose to get there, and sees no rat.

Stena: 22 (Invisible while in darkness)
Lia: 21
Not a Rat: 20 (-12 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15 <-----<<
Spiderbait:  15
Petmailer: 9
Galingal: 8 <------<<
Whatever is Shaking the Web: 6 (-8/??)
Jimjar: 4 

Galingal continues into the tunnel, following after Lia and Neth's voices, ready with his rapier if something that doesn't look like a rat gets close.

Galingal continues forward, alert for movement, passing by a short stalagmite, and peering up the tunnel, where he sees Neth and Lia looking back at him. They both give him confused and concerned looks, eyes darting around for the Not-Rat. Neth gives him a slow smile, and a thumbs up, mouthing <it'll be fine>.

Stena: 22 (Invisible while in darkness)
Lia: 21
Not a Rat: 20 (-12 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15 <-----<<
Spiderbait:  15
Petmailer: 9
Galingal: 8 (held rapier attack)
Whatever is Shaking the Web: 6 (-8/??)
Jimjar: 4 

Kolose, seeing movement, retreats further back into the tunnel. Moving thirty feet, and waiting for Stena to pass him by- Be it either visually or at least in sound, before he'd go further. But preparing a "toll the dead" should he see anything else.

Kolose moves away from Petmailer and Jimjar as he heads further in to join Galingal by the stalagmite, waiting for signs of Stena joining them.

Outside, Stena, having made it safely to the bottom of the rope, arms herself again after her climb.

"Stena? You coming?", Jimjar asks, backlit by glowing fungus, his eyes looking slightly to her left where the rope still sways. He holds his crossbow at his side, but looks ready to run as soon as he's sure Stena is following.

Stena: 22 (Invisible while in darkness) <------<<<
Lia: 21
Not a Rat: 20 (-12 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15
Spiderbait:  15
Petmailer: 9
Galingal: 8 (held rapier attack)
Whatever is Shaking the Web: 6 (-14/??)
Jimjar: 4 (held dash)

"Move before the spiders get brave again."

Stena orders at a loud whisper.

She'll move after them, if she is the last in, and will ready a bow attack at anything outside their group that moves. She does expect to see a not rock, not rat somewhere.

((Stena can get roughly to the mouth of the cave with a dash and still be at the rear of the party, or halfway and hold an attack.))

Stena moves halfway and holds an attack, particularly if that keeps her in the dark.

The 'stalagmite' that Galingal and Kolose are standing near shifts as a toothy maw opens up in what formerly looked like hard stone, but is now an amorphous, melty looking form. A sticky purple tongue lashes out at the elf.

Pseudopod Attack: 8 Damage: 6

Lia calls out "You're rats at being a rat!"

WIS Save: 15 or 4

The tongue flicks pack Galingal, who leans back just in time for it to impact the wall instead of him.

Neth sends a crackling blast of magic at the Not-Rat, who has revealed itself to also be a Not-Stalgmite.

Attack: 16 Damage: 10

((Galingal has a held attack, then his turn.))

Stena: 22 (Invisible while in darkness, held bow)
Lia: 21
Not a Rat: 20 (-12 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15 <-----<<
Spiderbait:  15
Petmailer: 9 <-----<<
Galingal: 8 (held rapier attack) <-----<<
Whatever is Shaking the Web: 6 (-14/??)
Jimjar: 4 (held dash)

Jimjar hustles into the cave.

Stena: 22 (Invisible while in darkness, held bow)
Lia: 21
Not a Rat: 20 (-22 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15 <-----<<
Spiderbait:  15
Petmailer: 9 <-----<<
Galingal: 8 (held rapier attack) <-----<<
Whatever is Shaking the Web: 6 (-14/??)
Jimjar: 4 

Galingal looks on as the disguised foot slaps out at him and smacks the wall.

He stabs forward with his rapier (Attack: 18 Damage: 7  )

Not sure what to make of the creatures, nor how dangerous it might or might not be, he stabs out at it again, and then again.

Action - Rapier - Attack: 23 Damage: 6 

Bonus Action - War Priest - Rapier - Attack: 8 Damage: 9 

Petmailer feels that he’s heard of something like this not-stalagmite, a tale from one of his fellow shadow sentinels, told around a campfire.  He stops his advance and pulls out his bow, sending an arrow toward the thing.  

Longbow : 25 to hit, for 5 points of piercing damage.  

Galingal ferociously stabs repeatedly at the thing, while Petmailer sends an arrow streaking into it. The creature seems almost startled by the furious strikes from the pair of elves.

Stena: 22 (Invisible while in darkness, held bow)
Lia: 21
Not a Rat: 20 (-51 HP)
Yuk Yuk: 18
Neth: 15
Kolose: 15 <-----<<
Spiderbait:  15
Petmailer: 9 
Galingal: 8 
Whatever is Shaking the Web: 6 (-14/??)
Jimjar: 4