Fianna collapsed when struck by the crossbow… The spectral blade easily strikes her and pushes her so much closer to death’s door…

At the start of her turn Fianna makes her own Death Saving Throw: 7
…

I believe Fianna is dead dead.  If an attack is made within 5 ft it’s automatically a crit and this loss of 2 death saves.  

Fianna collapses at the bottom of the steps, the spectral blade cleaving straight through her like an apparition. In an instant, she is gone. 

Fi'Finella becomes visible atop Fianna's shoulder and screeches – a horrible, high-pitched shriek that makes everyone pause for a moment. The little imp wails for a long second before streaking up the stairs and into the grey sky, then vanishing again. 

The robed figure in red licks his lips and mutters a satisfied "Mmmm." His face peers around the corner through the wind wall at Kelnan.

Ogmund shivers at the sound of the imp. DC 15 WIS Save: 19
...and finds that he can move once again. Kelnan's spell is broken.

Katernin and Molo may act next.

    Katernin: 18
√ Ogmund the spearman: 17 (no longer paralyzed)
    Molo: 13
    Ozymandius (Yellow Robe guy to SE): 12 (in stinking cloud)
    Last acolyte: 10 
x  Dain: 8 (sleeping and maul-less)
    Kelnan: 7
    Swordsman with short bow (NW): 8
    Three crossbowmen (incl woman with white hair): 7. (all in stinking cloud)
    Robed figure in red (NE): 5
    Fianna: 4

Molo is going to try to wake up Dain and hand him his scimitar, simultaneously inspiring him (d6 for attack, damage, save, ability check or AC)

edit: sorry a d8 now

Dain comes back to consciousness, groggy and confused. He accepts Molo's scimitar and looks around, bewildered at this tunnel of wind surrounding him, but empowered by Molo's act.

Ozymandius tries to compose himself in the potent ochre fumes. DC15 CON save: 16

The yellow priest waves his hands and mutters an arcane word. DC13 WIS check: 5 
...but nothing happens. A look of frustration crosses his face as he retreats deeper into the side room, shutting the door on his way in, still hacking and coughing.

The last acolyte throws himself fearlessly through the wind wall at Dain, his club held high.
DC 15 Strength save: 14 or take 10 bludgeoning damage.
...but is smashed against the ceiling by the powerful force. He is dead.

Dain, Kelnan, and Katernin may act next.

    Katernin: 18
√ Ogmund the spearman: 17 (no longer paralyzed)
√  Molo: 13 (scimitarless)
√  Ozymandius (Yellow Robe guy): 12 (in adjacent room to SE)
√  Last acolyte: 10 
    Dain: 8 (maul-less but awake, inspired, and scimitared)
    Kelnan: 7
    Swordsman with short bow (NW): 8
    Three crossbowmen (incl woman with white hair): 7. (all in stinking cloud)
    Robed figure in red (NE): 5
    Fianna: 4 ☹️

Kelnan sees another companion fall in front of him and scrambles back up the steps, away from the man with the terrible blades. Realizing his spell has failed, he attempts to slow Ogmund down a bit with a Ray of Frost.

Attack: 18 Damage: 13

Kelnan scrambles up and past Katernin, up and out the stairwell and out-of-sight of the man in red, but not before firing a frigid beam of blue-white light at the now-mobile spearman on the other side of the wind wall.

Ogmund takes the full brunt of the icy attack. He is in very bad shape, and now encrusted by ice, but furious and frustrated at the dangerous wall separating him from you.

Dain and Katernin may act next.

Dain swings his new scimitar back and forth a couple of times, testing the balance. He nods his thanks at the little kobold and steps up to the foe. 

Attack: 18 Damage: 7

Attack: 15 Damage: 6

Does Dain want to risk crossing the wind wall to whack Ogmund? (DC 15 Strength save or take 3d8 bludgeoning damage from the wind wall - half damage on success.)

(Pausing on resolving Dain's action until he's ready!)

The man with the bow to the northwest lowers it momentarily. He makes an arcane gesture toward Ogmund, but then releases a string of "sleeping dwarf" taunts at Dain. (Dain: Make a DC 12 Charisma saving throw or have disadvantage on ability checks, attack rolls, and saving throws until next turn.)

One of the stringy-bearded crossbowmen opens and disappears into Ozymandius' room to the southeast, shutting the door behind him. The other two crossbowmen ready bolts, waiting for a lapse in the wind wall.

The red-robed man directs his deadly blade toward Molo:
Ghostly blade attack vs: Molo: 6. Hit: 6 force damage.
...but Molo easily sidesteps the sloppy swing. 

Dain should clarify has action, and Katernin may also act! (Then, Dain should make that DC12 CHA save...)

   Katernin: 18
√ Ogmund the spearman: 17 (slowed by frost)
√  Molo: 13 (scimitarless)
√  Ozymandius (Yellow Robe guy): 12 (in adjacent room to SE)
√  Last acolyte: 10 
    Dain: 8 (maul-less but awake, inspired, and scimitared)
√  Kelnan: 7
√  Swordsman with short bow (NW): 8
√  Three crossbowmen (incl woman with white hair): 7. (all in stinking cloud; one with Ozymandius)
√   Robed figure in red (NE): 5
    Fianna: 4 ☹️

Dain goes from drowsiness to fury in an instant, tearing through the wind wall to get to Ogmund:
DC15 STR Save: 20 or take 11 bludgeoning damage
...(success, so halved and halved again due to Rage. 2 hp damage.)

Dain passes through the Ogmund takes a hit. The tough spearman is barely on his feet, but seems determined to see death before defeat.
The sturdy dwarf tries to ignore the taunts of the bowman: DC12 CHA Save: 11  
...but the magically-laced insults get under his skin. (Dain has disadvantage on ability checks, attack rolls, and saving throws until next turn.)

Katernin is up!

Ogmund, bleeding heavily and coated in frost, turns to Dain, laughs, and spits. Then he launches a fresh attack:

Spear Attack #1 vs. Dain: 12   Hit: 11 piercing damage.
Spear Attack #2 vs. Dain: 22   Hit: 13 piercing damage.
Shield Bash vs. Dain: 8 Hit: 10 bludgeoning damage (and Dain must succeed on a DC 15 STR save or be knocked prone.)

 The sturdy dwarf keeps most of his powerful attack at bay, but one spear thrust makes it through Dain's defenses (3 hp deducted from sheet), not that Dain can feel much in his enraged (and distracted) state.

Everyone may act!

   Katernin: 18
√ Ogmund the spearman 17 (slowed by frost)
   Molo: 13 (scimitarless)
   Ozymandius (Yellow Robes) 12 (in adjacent room to SE)
   Dain 8 (maul-less but awake, inspired, and scimitared; disadvantage on ability checks, attack rolls, and saving throws until next turn.)
   Kelnan: 7 
   Swordsman with short bow 7
   Three crossbowmen (incl woman with white hair) 7 (all in stinking cloud; one with Ozymandius)
   Robed figure in red 5

Molo says to Kelnan “Scratch out their eyes!” for a d8 inspiration added to attack, damage, AC, ability check or saving throw.

Then he proceeds to just make a trilling squawk as he casts shatter at third level centered on the door to the east where all those bad guys are

DC 15 Con for half of 22 thunder damage

Dain tries to stay on his feet (DC 15 Str Save):  11, but is knocked down.  However, it's simple enough for him to spring to his feet again.

Move:  half-movement to stand up from prone.
Action: Attack
1st: Attack: 14 Damage: 10
2nd: Attack: 21 Damage: 7
Divine Fury:  Extra 4 radiant damage on the first hit.

A deafening concussion impacts the east end of the room, centered on the coughing guards:
CON Saves: 9 and 10. (One of the guards had already disappeared into the room to the SE and shut the door; my apologies - I forgot to update the map.)

The blast is painfully loud and kicks of dust and debris, together with fresh waves of foul arcane gas. One of the choking guards collapses. The other (the older woman with the short white hair) still stands, barely, fighting for breath in the stinking cloud. The door behind is partially blasted open.

Dain just manages to get the scimitar past Ogmund's defenses and into the spearman's neck. But it's the divinely radiant surge that finally takes this grizzled gladiator down. Ogmund is dead.

Kelnan and Katernin are up next!

   Katernin: 18 (concentrating on wind wall)
√ Ogmund the spearman
√ Molo: 13 (scimitarless)
√ Ozymandius (Yellow Robes) 12 (in adjacent room to SE)
√ Dain 8 (maul-less but awake and scimitared)
   Kelnan: 7  (concentrating on cloud; inspired d8)
   Swordsman with short bow 7
   One crossbowmen (woman with white hair) 7 (in stinking cloud, badly wounded)
   Stringy-haired crossbowman in other room 7 (with Ozymandius)
   Robed figure in red 5

Kelnan tries to find an opening to reach the hooded figure at the base of the stairs, points his rod and tries to end this terrible fight.

Ray of Frost: Attack: 14 Damage: 10

Edit: Let's add that inspiration to the attack. +5

Kelnan descends the stairs until he can see the figure in red, then blasts a beam of blue frost at the hooded man. The hooded figure curses loudly in pain as his torso becomes caked with ice. (Is Kelnan retreating back up the stairs afterward or staying put?)

Katernin may act!

(Yes, Kelnan heads back up. The room is both dangerous and quite smelly.)

Katernin stands guard on the stairs waiting for the others to retreat behind her.

"Molo, Dain let's go," she calls out, diving energy in the words as she tries to help with Molo's wounds.

((Casting Healing Word (2nd level): 9))

Katernin successfully helps Molo with his wounds even as she tries to coax Dain back.

The well-groomed swordsman steps forward just on the other side of the windwall from Molo, and smiles confidently for a brief moment. Then with a wink, he detonates a concussive blast. A wave of thunderous force sweeps out from him. Molo and Dain must make a DC12 Constitution saving throw or take 8 thunder damage and is pushed away - Dain into the back wall, Molo into the wind wall. (On a successful save, they take half as much damage and aren't pushed.) Bodies fly, debris sprays, and a thunderous boom fills the air.

The coughing crossbowman tries to regain her composure:  DC15 CON Save: 17

 She manages to maintain her breathing long enough to loosen a bolt at Dain:
Heavy Crossbow Attack vs. Dain: 23   Hit: 7 piercing damage.

In a lucky shot, the bolt hits Dain square in the forehead. Crit damage: +4

The swordsman retreats back to his doorway.

The robed figure makes a gesture and, with a new, shimmering field surrounding him, retreats a few steps as well.
Everyone may act.

(Note: I'll allow Katernin to drop the windwall as a reaction to the blast if she chooses. That would: (1) Allow Molo an opportunity attack if he is alive and not thrown by the blast, or (2) negate any windwall damage if he is thrown, but also (3) remove the windwall barrier that has somewhat protected you until now.)

Molo con save -1