(Spar has 120ft of darkvision, and she has boots of spider climbing, so she will gravitate toward the front, though perception is not her specialty.)

Quote from Mingofaust >>

I suppose I should look at her spell load out, as she knew we would be going underground before the last Long Rest, right? So she could choose suitable spells?

(Yes, you were informed last evening that the search & rescue would be in the Underdark.

Since I know what will be coming up, I'll stay with Drusk's default spell list. If someone has a specific change to request, I'll do that.)

(Done, just swapped one spell out for Stone Shape instead.)

Jorin is happy to lead with the light of his moonblade allowing him to see.

I'd be inclined to have Quillary do the survival checks for flavor, I think Quillary would be into exploring the underdark, especially once wild mushrooms start appearing.

Before entering, Hildi (who has removed his leather armor) will cast mage armor on himself, raising his AC to 17. (I've adjusted his character sheet to reflect this.)

As for where in the line he should be, I'd say either last or next to last. I'm cool with Quillary rolling.

Let’s do some tracking!

Quillary’s first attempt at finding the trail. Survival: 18 

Quillary tastes the dirt as part of her tracking routine, because of course she does. She also lets a rat go from her hat, watching carefully as the terrified thing scurries away in a random direction.

Drusk will be last in line. His Sentinel Shield will help notice if anything is creeping behind the party.
Spar notices that the eye painted on Drusk's shield seems to be following her (like sometimes happens with old oil-paintings).
Whenever she looks more carefully, it is only paint.
The others don't react to the shield at all. (They are used to it, but Spar hasn't been around the party enough to know that yet.)

Once you descend past the levels of the Citadel occupied by the Lord's Alliance, you enter a completely different world. Without light, these tunnels press in on you, a presence that almost caresses the back of your mind with as you descend into the bowels of the earth. If the Zhentarim of the past delved too deeply and greedily, there is no telling what you might find down here skimming across the edge of the Underdark.

The air is cool and just slightly damp. The general impression is a faint whiff of rotting earth. (Quillary confirms the rotting earth with a slight taste, before spitting the dirt to the side.) The walls are solid rock, coated in many places with a faint sheen of mud. Occasionally slippery.. Sounds travel oddly and for a long distance underground. The tunnels are dark except for Jorin's moonblade.

Following the direction mentioned by the scouts, Quillary quickly finds traces of the large 4th scouting party: scuff marks on the stone floor from metal shod boots, crumbs from leftover snacks that the scavenging underground insects missed, etc. She leads you down a particular direction.

After 40 minutes, you come across a cavern (20' wide) that is filled with mushrooms, nourished by water slowly dripping from the ceiling. Despite Jorin's earlier jest (post 5873), Quillary says this is the right path, and not a personal distraction.

Before you enter the cavern, Drusk halts the party and points ahead. (He has the highest passive perception.)
Mushrooms fill the floor of the cavern for a distance of 20 or so feet from the entrance before abruptly stopping in a straight line across the rocks, then resuming 50 feet beyond the point where they stopped.

Quillary will put a hand to Jorin’s shoulder, to move in front of him. She moves ahead carefully, stepping between the mushrooms, approaching the dead zone, but not entering it. She will try to see or smell anything that can give her a clue as to the nature of the gap.

Perception: 15 

If that turns up nothing, she will gather her spores and send a cloud of mites into the gap.

If that turns up nothing, she will take off her hat, rummage in it and pull out a rat, whisper an apology to the poor, squirming thing, and toss it gently into the gap so that it lands on the clear ground.

If that turns up nothing, she’ll turn and shrug at the others.

(OOC: Normally that Perception roll is good enough, but I think it would be more fun to play it out.)

Quillary cautiously steps forward, stopping short of the no-mushroom zone. The rest of the party joins her.

She frowns. There is something wrong ahead, but she can't quite put her finger on it.
She wants to cast the Infestation cantrip, but the spell requires targeting a creature in sight.

After thinking a bit more, she reaches into her hat, pulls out a rat, and after a quiet apology, tosses it ahead.
After the rat crosses the "line", it falls to the ceiling instead of to the floor.
After that sight, you all recognize what had been bugging you subconsciously. The water drops on the floor are falling upwards. The 40' high ceiling is coated with mushrooms, extending from just in front of you to 50' away, where it stops (and the mushrooms on the floor resume).

Rats can survive a 40' fall without injury. You see the rat on the ceiling scamper through the mushrooms, away from Quillary. It passes that line 50' away, falls back to the floor, and escapes out of the cavern.

(I’m just excited the hat was useful in a game situation!)

Quillary still shrugs and looks to the others for ideas.

As the rat flies upward, Hildigrim can't help but exclaim, "Hainard's hairy toes!" He watches, big grin on his face, as the rat (faintly illuminated by the moonsword) runs along the ceiling and then falls back to the floor further on and disappears into the darkness. He walks over to one of the walls near where the mushrooms end — saying a quick incantation to bring his floating orbs of light into being to aid his inspection — and examines it for hand holds. "Perhaps we can climb across on the wall," he explains to no one in particular.

Perception: 25

Hey, DM, did you notice that Hildi as +8 on his perception as well? Plus, he has halfling luck. Does that give him a little edge over Quillary? Either way, he shouldn't be in front since he doesn't have darkvision, so she should keep rolling.

(OOC: Since Drusk has advantage on his Perception rolls due to his magic shield, his passive P score is 19, compared to 18 for Hildigrim and Quillary.
But you are right that the passive score doesn't account for halfling luck. Maybe some day I'll work out the math, and make a recommendation to WotC about how Luck should affect the passive scores.)

The walls are fairly slick due to water and mud.
Climbing the walls will require a DC 15 check for Athletics (STR) or Acrobatics (DEX) each time the gravity reverses (at the start and 50' later).
(The way the module is written, I think the idea is to climb to the ceiling, cross over the threshold, walk the 50', then climb back to the floor.)

(You have time OOC to consider more plans. Depending on your ideas, the DC can be lowered, or the possible fall damage on a failure would be reduced.)

Jorin is partially unnerved and partially in awe at the gravity reversal zone.

Is there anything nearby we could tie a rope off to?

Quote from Guiltysparc >>

Is there anything nearby we could tie a rope off to?

The trouble is the rock surfaces are slick with mud and water.

Jorin has enough strength that he could break off rocks where you are standing, and tie a rope to the newly-created anchor point.
That will take some time to hammer out. (Let's say 15 minutes, to make it clean enough to tie a rope and have it be very secure.)

Jorin offers that idea, thinking that maybe we can swing over (under?) the gravity field.

Are the surfaces of the cavern walls so slippery that they are effectively ice or coated in oil, for the purposes of slippers of spider climbing?

Hildigrim moves closer to the "middle" of the room and strings his glowing orbs out across the odd space ahead of them so the final orb is 60 feet away from him. Does everything he can see on the other side look just as wet and slippery?

Quote from DirtyDogP >>

Hildigrim moves closer to the "middle" of the room and strings his glowing orbs out across the odd space ahead of them so the final orb is 60 feet away from him. Does everything he can see on the other side look just as wet and slippery?

(Yes, wet and slippery for this entire cavern. But he doesn't see any more gravity-reversals.

He does see a rat  scurry further away into the darkness from the final orb.)

Quote from Guiltysparc >>

Jorin offers that idea, thinking that maybe we can swing over (under?) the gravity field.

Quote from rkmobius >>

Are the surfaces of the cavern walls so slippery that they are effectively ice or coated in oil, for the purposes of slippers of spider climbing?

(All OOC...
The walls are slippery, but having the magic slippers should be better than NOT having them...

Sounds like Jorin would like to anchor a rope in the magic zone, on the floor ("up" in the zone). Holding onto the rope, when someone crosses the line, they would swing "down" (towards the ceiling) then across the zone. When they reach the other side, they would cross that line and be back to normal gravity.
Spar would move across the line (using the magic slippers to stay on the floor) to 25' away, make a place to anchor the rope, then continue to the other side. It would be kind of like mountain climbing, using the anchor point and rope to prevent a 40' fall if the slippers slip.

This is my suggestion for how it would work in the game.
Feel free to try other ideas (if you don't want to do the swinging) or make your own suggestions, if this doesn't sound right.

Each player has a choice on how they want to swing across.
In all cases, the check is DC 15 for Athletics (STR) or Acrobatics (DEX), you get to chose.

• Be gung-ho. No matter what, you cross over, but on a fail you get disoriented or let go of the rope too soon, and crash into something on the other side. This will use up 1 of the 4 temporary healing potions the party was given, to restore the damage taken from the falling damage.
• Be cautious. No risk of damage, but you continue rolling checks until you succeed. Each failure wastes 10 minutes of time. (You have to get back to your starting point, get the rope, and try again.)

Spar has to do this too, but she is crossing over with the magic slippers and the safety rope.
She has to the same options as the others, but Advantage on the check.

Everyone should chime in before rolling the checks, to make sure if the party wants to do this.
It will use 10 minutes of time, no matter what, to initially get the rope swinging set up.
That still sounds better than the default, two checks with each having the risk of taking 40' of fall damage.)