Despite, or maybe because of, the rats swarming over him, Bryn is committed to trying to seal off the tunnel.

Strength: 6

(lololol… I guess Bryn better stick with feats of acrobatics or archery)

Ann will attempt to get Firth to the top of the stairs, emphasis on "attempt", with all the strength she has.

Strength Check: 20 (It could be Athletics, too, depending on DM interpretation, but that wouldn't change the result.)

Ann succeeds in getting Firth to the top of the ladder. How does she do this in a particularly impressive manner? Also, with a nat 20 I'd allow you to climb up with Firth and get back down in the same action if that is what you'd like to do.

Meanwhile, Bryn and Vedui are still frantically trying to dislodge the root in the roof, which remains firmly fixed in place. 

Jormund, what do you do this round?

Jormund will move to defend Bryn and Vedui from rats.

Assuming he can reach the swarm on his turn, Attack with warhammer: 24 Damage: 11 

Quote from rustleinthewoods >>

Jormund will move to defend Bryn and Vedui from rats.

Assuming he can reach the swarm on his turn, Attack with warhammer: 24 Damage: 11 

 Nat 20…. Nice!!!

Jormund, you deal a mighty blow to the rats and drive them back from your group. You have all collectively crushed, stomped, or slashed roughly 1/6 of the rats coming at you down the tunnel.  In addition, the rats are intimidated by your furious assault, they will not attack next round.  

Ann, let me know if you'd like to come back down to aid your allies.

NPC Round 4: 

The rats will swarm over Jormund, Bryn and Vedui, but are more concerned with their own safety at the moment and will dodge back and forth around your bodies.

What do you guys do next?

Bryn keeps tugging.

Strength: 5

(LOL.. this is getting comical now....)

Seeing that their efforts to collapse part of the tunnel were not successful, Vedui will turn to the rats yelling at them and performing a little stomp dance....

Attack: 25 Damage: 10

bonus action Attack: 25 Damage: 6

ok that's three combat crits in a row......dice roller is haunted....

Quote from Burgen_Badluk >>

ok that's three combat crits in a row......dice roller is haunted....

 (Time to buy some lottery tickets)

With Vedui and Jormund wading into the oncoming tide of rats like a couple of kaiju titans trampling a helpless city, the rats roil and recoil away in fear. They appear to be hesitant to continue the attack, at least for the moment. 

Bryn, Jormund, and Ann, what would you like to do this round?

Jormund uses the pause in rat approach to assist Bryn with whatever he is tugging upon. 

STR assist: 19 or add +2 if proficiency is included. 

With a gigantic swing of his warhammer, Jormund lends his strength to dislodging the root Bryn has been tugging furiously at. With a mighty blow, he knocks it loose, and with a mighty roar that portion of the tunnel begins to collapse!

Bryn, Jormund, and Vedui, please make dexterity saves to dodge out of the way of the collapsing debris. DC 10 or be knocked prone.

Collapse extent:
15

A cascading crack extends along the tunnel in both directions about 15ft. The area immediately becomes difficult terrain.

Collapse timing:

3

Round 1: Area becomes difficult terrain

Round 2: Every Creature in the area must make DC 15 Dex save to avoid falling debris.

Round 3: Every Creature in the area must make DC 15 strength save or become Restrained and Suffocating

 Rat Swarm Save: 4

Bryn Dex: 12

Bryn jumps out of the way of the cascading earth towards the ladder they came down previously.

Jormund attempts to scoot out of the way of the cave-in…

DEX save 22

Vedui attempts to dance back seeing the hammer fall onto the ceiling  "Ha, trust dwarves to know how to collapse a tunnel"

8

(Sorry, people warned me time passes quickly when you're having fun, but I didn't expect it'd also pass quickly when you're busy...  I'm going to address some earlier posts)

Quote from Naveyman >>

Ann succeeds in getting Firth to the top of the ladder. How does she do this in a particularly impressive manner? Also, with a nat 20 I'd allow you to climb up with Firth and get back down in the same action if that is what you'd like to do.

Ann takes a second to think, and her solution might not be what anyone expected. The truth of the matter is that in raw physical strength, the knight had nothing to show, but, bolstered by the determination to get Firth up the ladder quickly, she carries all of his weight on her back and climbs up with extreme ease. Once on top, she leaves him somewhere safe and practically slides down to return to the fight with her companions.

Quote from Naveyman >>

Bryn, Jormund, and Ann, what would you like to do this round?

Ann will start moving back towards the party, dashing if necessary. (The path to get to the stairs was long, if I remember well, so I'll not assume she gets there instantly)

... and seeing knight Ann coming back toward the earthfall, Jormund hollers, "Cave-in! Back to the ladder! Follow m'voice and move!"

Further saves...

Round 2 DEX save 6

Round 3 DEX save 17

Bryn additional saves:

Round 2 Dex: 18 (Doh! Well looks like Bryn is about to get knocked silly..... sorry guys.)

Round 3 Str: 9

Hey all,

Since you all except Vedui made the first save, if you choose to flee you can get out of the range of the collapse before the second round. 

Vedui, since you're a monk, I believe you can kip-up and flee as well to get out of the collapsing section as well using step of the wind.

Unless there is something you wish to do before retreating out of the collapsed section of the tunnel, you all find yourself watching as a long section of the sewer system collapses before you, accompanied by the frenzied, but soon muffled screeches of dying rats. No rats escape in your direction that you can see. What do you do now?

"So, Plan B then?" Bryn asks as they watch the dust settle from the cave in after fleeing to a safe distance.