If any of the rats are within range of Yevele, she will attack with her quarterstaff Attack: 25 Damage: 16 followed by a kick: Attack: 6 Damage: 4

ooc Natural 20 and natural 1 in the same round! And, I realized I screwed up the dice roll on the damage with the quarter staff. I did 2d8 rather than 1d8.

Smoke will be at the gate having taken one of the lead positions so wont be able to get to melee. 

Hearing the rats behind them Smoke will be quick to react doing a whisper almost to loud to be a whisper We have company!

He makes himself tall to look over the shoulder of those between Smoke and the incoming rats. Seems this tunnel is connected to a magic sewer because those turds are walking!

Action: cast Vicious Mockery on the first rat Smoke sees. DC 14 wisdom or take 2 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn.
Movement: none he will not be able to move much just turn around. 

Thia backs away, making space for people far better suited to be melee warriors than her.  "Oh, rats!  I hate rats!  Nasty, smelly things!"   Muttering the words of a simple cantrip she says, "Still, these rats are even more stinky and mangy than normal ones, I doubt they could harm a mouse, let alone our brave warriors!"

Move:  Retreat out of immediate danger.
Action:   Cast Vicious Mockery (DC 13) on the swarm of rats. Damage on failed save: 4
Bonus Action:  Use Bardic Inspiration on Thrax. 

Thrax steps in front of Thia to engage with the rats. His lizard like eyes focus in on one rat from the swarm and jabs forward with his spear. 

Attack: 7 Damage: 5

OOC: Sorry for any confusion, the rats are between you and the grate, so Rogram and Smoke would be closest, though there is enough space for Yevele and Thrax to make their way forward and be the melee attackers. Giving some more time for Rogram's action and possibly Dern's, though we may have lost our dwarven fighter.

Oh, well, in that case, Rogram bears his teeth at the oncoming vermin before bringing his sword to bear upon their tiny necks.

Two-hand longsword attack: 22; damage: 6 slashing damage

OOC: That would make three melee attackers, which is one too many for this bottleneck. Ordinarily I would take the first two attacks (Yevele and Thrax), but since there was confusion here, and Rogram is in front, any of the melee attackers can replace their attack with something non-melee.

Thrax will step back, and let his companions handle the little vermin. 

DM Screen:

Swarm of Rats WIS Save: 4
Swarm of Rats WIS Save: 10

Bites vs. Yevele: 21 Damage: 6
Bites vs. Rogram: 21

OOC: Almost forgot disadvantage on the attack against Yevele thanks to Smoke's vicious mockery: 9

True to their form, the rats swarm toward the party. Yevele's staff is well-placed, but ordinary weapons seem ineffective against the plethora of rats that emerge, only able to pick apart a few at a time. With Smoke's verbal help, the rats are slowed enough that she blasts through four of them in a single swing. Rogram manages to keep away from the other swarming rats for the moment, cutting apart one easily, but they seem less effected by Thia's mockery.

Round 2 Order:
Players <=====
Swarms of Rats

Placement:
The size of the tunnel prevents more than two melee combatants at a time.

Health:
Both swarms of rats are lightly wounded.

Rogram is lightly wounded.

Yevele will again attack with her quarter staff Attack: 13 Damage: 4 followed by a kick Attack: 13 Damage: 5

ooc Thrax, it might be useful if you moved up and replaced Rogram, because he is already wounded.

Thia continues to watch the combat, muttering words of power under her breath.  Seems pointless talking to these rats, they don't really understand me anyway.

Action:  Cast Vicious Mockery on the rat swam fighting Yevele.  DC 13 or 2 psychic damage and disadvantage.

Rogram swings again with his blade after dancing around a little bit to avoid being bitten.

Two-hand longsword attack: 12; damage: 5 slashing damage

Smoke will try and look down through past his companions to see the swarm taking out one of his daggers. Thinking if he throws it at the swarm it must hit something. 

He draws back his arm and with a swing tosses the dagger towards the swarm near Rogram. 

Action: Throwing dagger the swarm near Rogram Attack: 27 Damage: 8
Movement: Only stepping aside if Thrax passes. 
(wow a critical 1 and 2 :P)

OOC: Botting Thrax for this round with dodge.

DM Screen:

Swarm of Rats WIS Save: 2

Bites vs. Rogram: 10

Between Yevele's staff that crunches through another pair of rats, and Thia's mockery that drops another dead in its tracks, one swarm of rats has had enough. They turn tail and flee, exiting between the holes of the exit grate and onto the banks of the stream beyond. The other swarm is still attacking in a frenzy, but Rogram slices through one rat as he continues to step around the incoming creatures, while Smoke skewers a pair with a dagger like a disgusting kebab.

Round 3 Order:
Players <=====
Swarm of Rats

Placement:
The size of the tunnel prevents more than two melee combatants at a time.

Health:
The swarm of rats is lightly wounded.

Rogram is lightly wounded.

Smoke smiles as one swarm runs. Looks like they are turning tail he says as he holds his own tail in one hand waving it around. 

Taking another dagger with his free hand he waits for a good opening to and throws the dagger at the swarm. 

Movement: none
Action: Throwing dagger at remaining swarm: Attack: 25 Damage: 6

Yevele will once more swat at the rats with her quarterstaff Attack: 19 Damage: 8 followed by a kick Attack: 6 Damage: 5

Feeling a little heartened, though surprised that besting vermin could bring so much satisfaction, Rogram once again swings his sword low.

Two-hand longsword attack: 8; damage: 4 slashing damage

Thia continues to mutter under her breath, pulling mystical words from the Void to debuff the rat swarm.

Action:  Vicious Mockery.  DC 13 Wisdom or 1 psychic damage and Disadvantage on the swarm's next attack.

OOC: Botting Thrax again for this round with dodge.

DM Screen:

Swarm of Rats WIS Save: 15

The last of the rats scatter once Smoke throws a dagger into one, and Yevele swats two more dead against the wall of the tunnel. The few that remain are too quick out of the grate for Rogram to reach, and Thia's words have little effect as they reach the banks of the stream.

The dust settles upon the cramped and unexpected battlefield.

OOC: Each character has received 20 XP for chasing off the swarms of rats.

Area Description:

A thick iron grate blocks the exit from this tunnel to the stream. A large corroded lock is holding the grate in place.

Seeing the last of the rats flee Smoke sighs, Good riddance. He leans against the wall for rest a moment but quickly jolts back away from it realising its full of spiderwebs and other filth. He shivers thinking about it and wipes himself off a bit again. 

Assuming Yevele will finish her earlier attempt to use the key on the door Smoke will try to collect his two thrown daggers from the floor. With a disgusted look he peels off a dead rat and wipes it down best he can before tucking them back on his belt.