Just before we reach the top of the stairs, Mana summons the friendly shadows to obfuscate their presence. Hopefully the half-cover (+2 AC) will come in handy as we step into the fray.

A lot is happening in an elegant foyer you have entered at the top of the stairs:

A battle rages between several Zhents and house guards, and the floor is strewn with dead bodies (see below). Due to the shadows, neither side has immediately noticed you. 

Doors to the master bedroom stand open. From within you hear a lady shout “The City Watch is on the way!”

Another door is ajar. Beyond it, you can hear someone putting a boot to another door.

Lying on the floor are the corpses and weapons of six guards wearing House Gralhund livery over chain shirts and two Zhents in black leather armor.

Fight in Progress. Four veterans of House Gralhund are fighting off an attacking force of Zhents consisting of three thugs. The Zhents are trying to get to the master bedroom but the guards are blocking them. Despite heavier casualties so far, the guards seem to have gained the upper hand now

Oriace takes in the carnage in a heartbeat, then steps towards the nearest Zhent with quiet certainty. “House Gralhund—need a hand?” he rumbles to the embattled veterans, and without breaking stride pitches his voice to carry like a watchman’s call (Deception 13, influence action): “The City Watch has arrived! Stand down!” The bass-rolled bluff lands as his rapier does—Oriace glides to the nearest Zhent, blade flashing for a clean, decisive thrust, trying to use the confusion his shout sows at least to drive the attacker back from the bedroom doors. 

“Awww, hells no. This is a fight you cannot win!” Mana shouts at the thugs.

He brings his rapier up in a subtle salute while his other hand traces an arcane pattern in the air. His voice chirps slightly as he draws upon the Weave to curse the three thugs.

Bane

DC 14 CHA save for each of the three or -1d4 on all their to hits and saving throws for the next 1 minute or loss of concentration.

Wagner dashes in and unleashes a slashing flourish against the thugs. [[AC 18, 17 piercing, 6 piercing on a second thug]]

"Your cause is lost! Stand down!"

Oops, for some reason I was waiting for a post to be made. I thought I had resolved this round. To make up for it, I also attach an updated map!

Oriace, you feel like some of them seem to believe you. It emboldens the guards, but the Zhents seem determined to fight even if you are the watch.

Mana, Bane affects 2 of them. (We will call them A, B and C for targeting. B was unaffected)

Wagner, you come and swipe against A and B. A is on death's door. B is looking injured (well, officially injured of <50% health. They didn't start this on full health and neither did the guards).

A and B retaliate on Wagner who looks squishier than the highly armoured guards (despite having the same AC due to high dex). They swing their maces, and both miss (12 (14-2) and 14). C is in combat with 2 guards so keeps attacking them. They deal damage to Guard C who is now also officially "injured".

The guards then attack. Guards A, B and C attack their relatively named thug. D attacks C.

Thug A dies. Thug B is very injured. Thug C is just about injured.

You are all up.

Wagner just makes a calculated strike on Thug B. 

Attack: 18 Damage: 10 piercing

It had 10hp remaining. It now has none 

Mana will maintain his concentration of Bane on Thug C. If the way is clear enough (suffer no opportunity attack), he will move to peer through the door to the South, rapier still in hand. The boot to the door sounds beyond concern him. He will Ready a Vicious Mockery to trigger upon identification of the strongest opponent in the southern room. “A broom salesman knocks harder than you!”

DC 14 WIS or 3 psychic damage and disadvantage on next attack.

Oriace ghosts out of the dancing shadows, circling the last Zhent with small, sure steps; his deep voice cuts low and calm—“Yield”—then he feints high, drops his shoulder, and drives the rapier in hard at the thigh, angling to hamstring and end the fight fast.

Action: attack the last fighting Zhent, hits AC 22 for 8 damage.

Mana looks through the door and it is a bathroom. A curtain to the east has been drawn back, revealing a claw-footed bathtub. There is a further door to the south with the single occupant of the room, a large human male (image in spoiler) who is attempting to kick the door down. He casts his vicious mockery but it gets resisted.

Oriace gets a solid hit in, leaving the thug critical.

 Thug C attacks Oriace in retaliation, not yielding as instructed, just hitting (16) for 5 damage. The guards then finish off that thug.

The newly spotted thug notices he is not alone, runs out, taking a quick attack at Mana as he passes. First attack is a miss (14) but the second is a hit (25) for 10 piercing damage. He then dives through a window in an attempt to get away. (Mana and Wagner will both get an AoO) Everyone is up.

(I believe the entire foyer would be inside my 20’ radius Dancing Shadows, thus Oriace’s AC should be still 18. But that is your call.)

Mana yelps in pain. He parries the first attack but gets clobbered by the second. (My reaction was already taken by taking the Ready action, but I would love to attempt to trip (Shove Prone) him, if given the opportunity.)

”He’s getting away!” He shouts.

I did totally forget about dancing shadows. It does miss. And I'm not counting that as a ready action. You moved to look inside and could just take an action to cast vicious mockery. No ready needed 

That’s awesome!

As Mana sees the thug breaking towards the window, he’ll try one of the following opportunity attacks. If the DM allows the Unarmed Attack of shove (trip to prone), Mana will try to use his small frame to push him off balance enough to fall down or redirect him enough to miss the window opening.

DC 10 STR or DEX save (thug’s choice) or prone. (Assuming 8+prof bonus+ Mana’s STR)

If it must be a melee attack, Mana will strike out with his rapier.

Attack: 14 (9 advantage utilizing the Bardic Inspiration if 14 misses) Damage: 4 piercing (lol) if hit.

2024 phb states explicitly for an aoo you make a melee strike or an unarmed attack. So shove is valid. Unfortunately that's an 18 dex save so does not trip them. This one seems a lot more lithe (and seems a lot more battle hardy) than the other more grunt ish thugs. (if you couldn't tell by the fact they had artwork). 

[[AC 26: 11 piercing (Defensive flourish +1 AC)]]

Well shoot. I’m taking back my Bardic Inspiration, then.

That hits. The Zhent jumps out the window, and is clearly hurt by the fall but doesn't look too injured in total. He gets a total of 20ft away (including the drop to the ground from the window). You are all up.

Oriace’s thoughts flicker to Mana and Wagner—'This one matters; he may have answers. I’m on him.'—and his bass voice rumbles to the guards: “Secure the floor!” He snaps his rapier into its frog, vaults onto the sill, and launches after the fleeing Zhent—cloak flaring as he tucks into a tight halfling dive, aiming to roll on impact and surge up in the same breath (Acrobatics 22). Coming out of the tumble, he drives low at the man’s hips, trying to cinch both arms around the waist and hook a boot behind the thug’s ankle, intent on locking him in place and dragging him to a halt. (Unarmed Strike Grapple DC 10, would use Silvery Barbs on successful save. I'm unsure if the man is prone from the fall and whether or not Oriace managed to avoid fall damage with his maneuver.)