When Braxiel heard the noise he will cast mage armor not to be caught off guard. This would have been done before Brynn went towards the door. 

New AC is 15

Brother Coltinar,  "Ugh..Foul creatures."

Borther Coltinar on his turn....

Action:  Cast Bless and target Brynn, Rylia, Braxial

Bonus Action: None

Movement:  Move to be behind his friends but out of melee range himself, as much as possible.

Reaction: Opportunity attack if it is available.

End Turn:

If the rats perish before Brother Coltinar's turn, his action will be modified to heal the injured.

Rylia had listened to the shouting and her companions' reactions, not really understanding what was happening at first. Finally as the rat comes after Arr she does catch on and readies herself to help out her friends.  

(OOC - just catching up and will take action once others have acted) 

(All rolled in game log =)
Reaction: AoO: 19 Damage: 5 
Action: Rapier attack on same giant rat: 25 Damage: 10
Bonus action: Offhand attack w/ dagger: 21 Damage: 3

"Watch out! Giant rats!" he calls out as he takes three quick strikes in rapid succession, then steps around to make room for the others to come down the steps...

While Paladin rushes down to the basement, it is possible to feel the dust fall through the gaps in the wood floor caused by his strong steps. He is followed by Arr and Brynn.

Braxiel heard the noise and his friends running and immediately cast words of protection not to be caught off guard.

Arr, despite having promptly gone down following Paladin to fight whatever it was, are still trying to understand what is going on when they are attacked by a giant rat that appears out of the blue on their right side and jumps to bite their right leg. They feel the pain of the bite and see that the dark and sticky liquid that oozes from the animal's teeth now penetrates their leg, although without any effect since the fluids that circulate their body protects them from any disease.

Brynn, better positioned, and seeing Arr being attacked, takes advantage of the beast's proximity and delivers two accurate blows with his rapier almost slicing the animar's throat, who instantly falls dead. He still tries a third blow with his dagger, but fumbles while holding the weapons and is unsuccessful.
(--- DM you have no extra attack as a bonus action, so I will consider only two attacks, ok?)

Seeing that their friends are now fighting with diseased beasts, Coltinar and Rylia position themselves on the stairs trying to be within reach to help, but outside the combat area.

Braxiel is still on upstairs.

//--- Initiative Rolls---//
Brynn
arr --> next
Rats (1)
coltinar
Rylia
Rats (2)
paladin
braxiel
Rats (3)
rust

(Ah, dagnabbit! I keep forgetting you can't do two-weapon fighting with a rapier!! Sorry =)

Quote from Rhanloi >>

(Ah, dagnabbit! I keep forgetting you can't do two-weapon fighting with a rapier!! Sorry =)

 You can with Dual Wielder feat...  Grin.  Later on.

Move: move to rat 1 (or whichever is nearest)
Free Action: Reckless Attack (When you make your first attack on your turn, you can decide to attack recklessly, giving you advantage on melee weapon attack rolls using STR during this turn, but attack rolls against you have advantage until your next turn.)
Action: Attack (meat cleaver/handaxe) 18 for 9 slashing damage
Bonus Action: Attack (broken pipe/light hammer) 23 for 1 bludgeoning damage

Arr stares a moment at the dead rat that attacked them seconds ago, before they launch themselves recklessly at the nearest rat with a double strike.

The second Rat (1) sees Brynn knock its partner down in two killing blows and lunges forward to attack the changeling.

Attack: 11 Damage: 6

The Giant Rat jumps to bite Brynn in the arm, but misses the target and hits the armor at the height of his chest, which makes the beast to lose balance and practically fall at Arr's feet, who, with their "meat cleaver" in fist crushes its head.

From the back of the room you hear Rust's female voice yelling, "A little help here?" And Brynn, Arr and Paladin can see the fallen Warforged trying to get rid of the two rats that surround her.

Coltinar who is a few steps before the end of the stairs, sees that Brynn was attacked and casts an aura of blessing to protect the young man, Rylia, who is a little further in front of him, and one of his other friends.

// --- DM I may allow Coltinar to revise the other bless according the combat moment, if you wish
//--- Rats (1) are deads. You have a pair of rats in each Rats turn

//--- Initiative Rolls---//
Brynn
Arr
Rats (1) both deads
Coltinar
Rylia --> next
Rats (2) approaching stairs to try to go up
Paladin
Braxiel
Rats (3) attacking Rust
Rust

Brother Coltinar will change to himself for the bless considering the tactical situation.

Rylia stays in her spot on the stair, trying to keep back from these rats. She calls out arcane words, moribund with her hand towards a rat approaching the stair.  A beam of blue-white light heads towards the creature. (Ray of Frost)

(Attack: 22, Damage: 2; if hit, target's speed is reduced by 10 ft until my next turn)

Braxiel will move to the top of the stairs and and summon a small black bell in which he taps with his hands, everyone does not hear a sound except for the Rat that Rylia hit.

casting toll of the dead the Rat must make a wisdom saving throw of 13 or take 2 of damage

(Posting Bryn's action now since it will probably be his turn in the middle of my night =)

One of the rats comes undaunted towards the stairs when a freeze ray coming from that direction hits it. He feels his body lose energy and slow down, but he still finds the strength to attack Brynn, who is nearby, trying to bite him on the leg.
Attack: 24 Damage: 10

The second mouse comes to the aid of its partner, but first it jumps to try to bite Paladin in the neck.
Attack: 23 Damage: 6

Paladin swipes his shield up and then around to try to press the rat against the wall, then sweep in with his hammer to try to flatten it

Attack: 25 Damage: 16 

A little dizzy, the first Rat walks right past Brynn and bites the foot of a small table that is still standing with all the mess.
The second mouse, eager to help his partner, misjudges his jump and fails miserably to attack the Paladin.

// --- DM ---//
- Arr have an opportunity attack on the second Rat, assuming they were flanking Paladin.
- Considering that it is a spiral staircase, I would say that, on top of the stairs, Braxiel can´t see the rats.

Thinking in something like this:

//--- Initiative Rolls---//
Brynn
Arr
Rats (1) both deads
Coltinar
Rylia
Rats (2) approaching stairs to try to go up
Paladin  --> next
Braxiel
Rats (3) attacking Rust
Rust

As the rat moves away, Arr brings down their cleaver.

Attack: 20 Damage: 7

Seein the Rat passing by toward Brynn, Arr rotate their cleaver hitting the animal that moves forward staggered reaching the botton of the stairs.

Paladin does a pirouette to hit the beast and in that move, that for some reason comes out clumsy, he feels a strong stab in what would be his ribs. He tries to canalize that pain, swiping his shield up and then around pressing the rat against the wall, then sweeping in with his hammer to flatten it until sees that the Rat is dead.
//---DM Paladin takes 2 piercing demage ---//

Braxiel, trying to follow the voices downstairs since he sees nothing from where he is, deduces that the rats are trying to reach the stairs to move up. He tries to help casting some magic words in the direction where the animals should be, but everything is moving very fast in this semi-darkness and he doesn't succeed.

At the back of the room, the pair of rats that knocked Rust down will attack her.
First Rat Attack: 24 Damage: 10
Second Rat Attack: 20 Damage: 6

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Rust action athletics 23