Chives manages to twirl the mop without having it slip. She doesn't look very elegant though.

“Let’s do it. Let’s see if the traps are worth their salt.”

After a few minutes of walking, you all find yourself in front of the closed cellar door.

"Are we ready?" If everyone is ready, Chives opens the door and get readies to whack some rats. As the door opens, she begins singing a jaunty tune, thoroughly out of tune.

William watches the door open, axes at the ready. 

The door creeps open and...

Nothing. The darkness of the basement stares back at you.

Chives looks around, confused. "Hmm... That's strange."

Perception: 10

 Chives, with her darkvision notices the form of countless rats moving across the floor. There's a swarm of rats down there...

17

The rats notice you and squeak in unison. More rats dart out from the shadows and look to escape from the basement and attack you.

Roll initiative (or come up with something to not get eaten by rats)

“WOAH!” William yells as the rats dart from the shadows and he hears them squeak... he attempts to muscle the door shut on them.

Athletics?: 9

initiative: 10

Initiative: 8

Chives is excited to heroically defeat the ignoble plague of vile rats with her valiant weapon, the broom. So much so that at first she doesn't notice William pushing the door shut behind her.

(With that low roll...) William kind of trips over his feet trying to shut the door and attempts to play it off like he didn’t really mean to shut the door... “oh Chives, I’m so sorry!” 

(Didn’t realize you were in the stairwell...lol!)

Chives has the door shut behind her. William falls on the ground and gets back up trying to shut the door. Joben fires an arrow that flies past the closing door and hits a rat and kills it.

Initiative of rats:

Rat1 20
Rat2 8
Rat3 13
Giant Rat 8
Swarm 10

MAP:

                    = = = = =
                    = - - - -
= = = = =           = - - - -
- - - - = = = = = = = - - - -
- J - W # C @ S S \ \ - G - -
- - - - = = = = = = = - - - - 
= = = = =           = - - - - 
                    = - - - -
                    = - - - -
                    = - - - - 

Scale is 5ft. Wall (=), Door(#), Stairs(\), Floor(-), (@) contains 3 rats: 1, 2, 3)
Things in bold are active creatures or targets. Things in red are dead.

Rat1 rushes Chives and bites at her ankles.
Bite: 6 Dmg: 1

The Giant Rat stands its ground.

The swarm tries to overwhelm Chives, crowding around her.
Bite: 14 Dmg: 3 (It's a tight space)

The Rats 2 and 3 move in beside Rat1 and begin trying to climb Chives, biting and scratching her in the process.
Bite2: 8 Dmg:1
Bite3: 4 Dmg:1

Chives stands her ground and suddenly feels a sharp sting in her toe. She looks down to see a rat biting down on it. More rats try to climb her legs but she shakes them off. Before she knows it, she's surrounded by rats. She gets bit 3 more times in the span of a couple of seconds.

William, Joben and Chive's turn.

Chives yelps, and swings at the swarm of rats.

Attack: 14 Damage: 3

Chives swings wildly and manages to knock 3 rats away like golf balls. More rats move to replace them though.

If you want, you can make an Investigation(INT) check to determine how many rats constitute the swarm (The monster's current HP).

MAP:

                    = = = = =
                    = - - - -
= = = = =           = - - - -
- - S W # = = = = = = - - - -
- J S S @ C G \ \ \ \ - - - -
- - - - = = = = = = = - - - - 
= = = = =           = - - - - 
                    = - - - -
                    = - - - -
                    = - - - - 

Scale is 5ft. Wall (=), Door(#), Stairs(\), Floor(-), (@) contains 3 rats: 1, 2, 3, 4)
Things in bold are active creatures or targets. Things in red are dead. Things in orange are dying (rolling Death saves)

Joben and William's turn

(If Investigation is a free action this round, I'll do it.)

Investigation: 19

(It is)

You notice that there are 23 Rats in the swarm.

Joben, the door is closed. You can't see anything that's happening to Chives or the rats. You can just hear them.