(For movement, Fran and Brutus would keep with Lulu if possible for that round.)

Adray will shout out, "To Ordrag!"  He will then rush to aid his friend while he taps into the the power of the Dragon Queeb to better defend himself.  

Move and dash action to get 100 feet closer to Ordrag.  

Bonus action to cast shield of faith upon himself to raise AC to 21 

Joyning that battle was an obvious risk yet no one hesitated. Adray run to Ordrag, straight to the middle of it, but at the moment demons were busy with their more dangerous foe than a few mortals. Yeenoghu attacked Zariel:

19 Damage: 11 + 19

19 Damage: Unable to parse dice roll. (paralyze 17)

21 Damage: 17 (confuse 22)

The flail in the clawed hands of the demon raised and dropped three times when a trumping sound covered the battlefield, overcoming deafening noise of the battle  "I am here, Zariel! I am coming to help!" Lulu rushed to the middle of the combat ignoring everything, straight to the monster attacking her dearest soul in this universe. And she was changing as she was moving - little fluffball turned into the huge golden fur mammoth, still flying on her - now huge - wings. She slammed into the demonlord:

Attack: 12 Damage: 31 (save: 28 )

Unexpected demon fell to the sudden attack and Lulu dropped all four legs on him: Attack: 26 Damage: 35 ( 14 )

Mooncolor - hesitant at first atthe sight of the battle - finally made his choice and jumped into the fray too, attacking the frost-demon: 25 (save 11 )

That... could be considered a miracle - the demon, distracted enough by the battle, did not even pay attention to the big, but mere a minor celestial, and the unicorn did not really attacked, but just touched the demon with the horn. The giant only begun to turn his ugly head to look at the sudden interference and opened his mouth either to laugh or to curse, all sure in his power and invincibility - but the light from Zariel's (no - Ordrag's!) sword and the shimmering shield around Mooncolor collide into a flash of painfully bright celestial light, that consumed the demon. When the light faded away, the spot was emty - the was no lake of ichor or anything proving demons existence, as if it just disappeared without a trace.

Now it was Zariel - and the party! - against one demonlord. But that demonlord was not going to back off.

Zariel, taken aback by Lulu's appearance and the light show behind nevertheless did not loose focus and stroke Yeenoghu again:

• Attack: 21 Damage: 20 + 39fire
• Attack: 28 Damage: 23 + 31fire
• Attack: 20 Damage: 18 + 32 fire + 5 cold AoE

The demon - wounded and charred - snarled trying to bite those standing around him:

• Attack: 25 Damage: 13
• Attack: 26 Damage: 18
• Attack: 21 Damage: 11

Everyone!

    Polly knew her darkness was likely useless yet she just felt more comfortable with it surrounding her in the presence of such powers.  Perhaps it was from being little so long, but being hidden was a comfort and if the darkness didn't really work she didn't have to focus on that... "Well OwImp, this is likely it...  Let's go out blasting, huh?"

     Her familiar had nothing to add except a squeaking noise and pulling the flap of the backpack closed and hiding away in there.

      Polly moves to J16 and cuts loose with the Eldritch Blasts towards Yeenoghu.

EB 1 Attack: 18 Damage: 6

EB 2 Attack: 17 Damage: 11

EB 3 Attack: 15 Damage: 11

 ((lol, if I can't roll higher than an 8, I suspect Polly will mainly be an observer. *sigh*. Dice hate me.))

Choir casts Call Lightning.  Yeenoghu is so large, she is pretty sure she can hit him without risking damaging friendly fighters.  Just to make sure, she calls out an order in Auran to her myrmidons to get clear of the gnoll demon, before calling down a lightning bolt to scorch his behind for 28 damage. (Dex save DC18 for half damage)

(Fran move to J14? Brutus to Adray’s target if he can dash make it.)

Fran would run in after Lulu, in awe of her transformation. Cocking her crossbow, the artificer would take careful aim and fire off a succession of bolts to Adray and Ordrag’s target.

Attack 1: 22 Damage: 7 Bless: 2 
Attack 2: 25 Damage: 7 Bless: 4 
Haste Attack: 19 Damage: 12 Bless: 4 

Brutus would dash his way to stand with the closest target.

Ordrag shouts in triumph at the demise of the demon lord, not in glory for any personal victory but for the united effort of the assembled host. Turning to assess the other demon lord he notices Adray's arrival, and as he did before with Polly, he reaches out a hand to offer to fly Adray into the fray to fight together side-by-side.

(Can Ordrag fly to the far side of Yeeoghu and pull/lift/piggy-back Adray as well? If so, does that constitute an action?)

(That would be an action and Ordrag will be heavily encumbered (half distance?) but still should be enough to reach the spot, I think?)

"Heavily encumbered" normally drops speed by 20 feet, but even if it's halved, can still reach far side of demon. Let Adray confirm that he's okay with that first? Unless he has other plans or options. I didn't want him stranded for a round with nothing to do but Dash.

(Let's assume Adray did not object the transportation)

Ordrag and Adray softly lowered behind the giant hyena lord. The glowing (and most likely hurting too) sword would have to attract demons attention if it was not too busy fighting the bigger foe. Zariel used even the slightest distraction - her fiery gaze burned the demon again: 22 (32 )

For all his obvious might, you knew from before, Yeenogu was not invincible. Choir proved that once again, summoning the lightning on the head of the demon lord: (save 26 ) It did not run - not right away - but it howled madly and tried to bite everyone it could reach:

vs Ordrag Attack: 25 Damage: 18

vs Adray Attack: 19 Damage: 16

vs Lulu: Attack: 27 Damage: 17

vs  Zariel Attack: 18 Damage: 16

vs Mooncolor: Attack: 29 Damage: 12

Then Fran struck, hitting with every bolt sent. The demon did not fell still but those bites were definitely felt by him and Zariel made sure to add more fire to the mix: 16 (33 )

Lulu's angry trumpeting (21 ) topped the massive attack.

Polly's bolts missed in that hurricane of attacks, but Yeenogu did not wait for everyone to course correct. He suddenly disappeared but only to return a step behind Mooncolor. The brave Unicorn used that opportunity to touch the fiend: 22 but the miracle (may be in the absence of the sword) did not happen again, the demon answered with the flail:

Attack: 18 Damage: 14

leaving three long bleeding open wounds across unicorn's back (16)

Mooncolor was still standing if barely but these wounds bought time for Zariel to take the position:

• Attack: 21 Damage: 21 + 45fire
• Attack: 29 Damage: 19 + 46fire
• Attack: 23 Damage: 16 + 23 fire + 11 cold AoE

Three very precise strikes visibly shattered the fiend, it looked as if the slightest push could destroy him now. For the first time you saw the terror in the eyes of the high demon - unable to run, it was awaiting its fate now.

 Everybody!

     Polly drops her Darkness and flies 30 feet towards Yeenogu (to I-15), she's not important enough to be targeted here anyway...

     "We've already seen this battle, you're already dead..." Polly says, likely out of earshot of anyone but OwImp.  Still, it was within those words and the subtle, precise hand movements that she made, that she cast Toll the Dead upon Yeenogu...  ((Wis save vs DC 18 or 29 Psychic Damage.))

On the cusp of victory, Ordrag shouts out a prayer to Tyr, flies directly at Yeenoghu, and brings Zariel's sword down in a great overhead swing, and then spins in the air for another sweeping strike.

Bonus Action: Cast Blinding Smite to deal an extra 12 radiant damage on the next successful attack. Additionally, Yeenoghu must succeed on a Constitution saving throw or be blinded until the spell ends. 

Attack #1: 11 Damage: 15 + 21 radiant damage
Attack #2: 17 Damage: 8 + 25 radiant damage
An evil creature that takes this radiant damage must succeed on a DC 17 Constitution saving throw or be blinded until the end of its next turn.

If either of the above attacks hit, also use a 3rd-level slot to do Divine Smite for an extra 17 radiant damage

Choir will fly closer to the battle--near Lulu--and call down another lightning bolt from her storm, either on Yeenoghu, if he is still standing, or any other likely enemy target. 35 damage. (Dex save DC18 for half damage)

Should the demon lord remain standing, Fran would take careful aim with her crossbow once again. She'd fire off the required shots needed to take him down before moving on to Yeenoghu.

Attack 1: 24 Damage: 10 Bless: 4 
Attack 2: 26 Damage: 8 Bless: 3 
Haste Attack: 26 Damage: 13 Bless: 4 

The moment one of her bolts would strike a target, Fran would stomp her feet to activate an electric charge that'd spark in an arch towards Mooncolour. It'd give him a little boost!

Arcane Jolt Heal Mooncolour: 8

Brutus would begin bounding his way towards the foe with the others.

Adray takes a few quick breaths and then takes a small hop.  As his feet hit the ground he was running toward Yeenoghu.  He crouched low and drew lightning to his fists and then jumped striking out four time as he traveled higher.

Move to Yeenoghu

Action: 2 attacks doing lightning damage

Bonus Flurry for 2 more attacks

Attack: 26 Damage: 7

Attack: 11 Damage: 8

Attack: 22 Damage: 10

Attack: 30 Damage: 7

It was one mighty tremendous final attack from every side, every angle, with magic and steel alike. Dolorous bells and thunder accompanying the smites of lightning, holy light and barrage of more material strikes played a the last fitting accord signifying the death of the Beast of Butchery, the mighty Gnoll Lord.

The demise of the demon's leader was met with the loud howling, shrieking and roaring from both sides, but demons, though not running away, took an obvious blow to the morale at least. And yet nothing changed for Elturel - the city was bout to get drawn and quite soon. Panicking screams of the people above could be heard now even here, through the all battle noises.

Zariel glanced at the city the way someone usually observes a work progressing according to plans and turned to the party, lowering but not sheathing her weapons. The fiery gaze stared coldly at Lulu.

"You have returned. Why? And you," she looked at everyone, meeting the eyes, piercing though the very soul, "you helped to destroy that demons. Not a small task for a few mortals, even the one with MY sword. Why are you here?"

     "Well I have these theater tickets to a play I've been really excited to see... But... well...  errr...." Polly gestures towards the city then shrugs.

     Polly didn't try comedy often.  That was probably a good thing.

Choir lands at the fallen angel's feet, still maintaining her storm cloud above.

"We have come to save Elturel and return it to where it belongs," Choir says firmly. "This is our mission, to save Elturel...above all!"

She stood proud and fierce, facing Zariel. It was nice to be able to speak for herself for a change.

Ordrag moves forward toward Zariel, and stands next to Choir. Softly he says to the kenku: I am grateful for your presence. Will you grant me Akadi's blessings? He then turns back to Zariel and says boldly in Common:

Great and mighty Zariel! Long have you ruled over Avernus, holding at bay the demonic hordes. You have given much, indeed all that you once stood for, in order to stem the tide of chaos and save others from the demonic terrors of the Abyss. But now (and Ordrag points toward Elturel slowing sinking into the Styx) you seem to have forgotten even this final virtue, and you would sacrifice the lives of many innocents for the sake of victory.

Then switching to the Celestial tongue and raising his voice, Ordrag says: A fell Choice you made then. Now is the time for Choice again. Here is thy sword, Zariel! Take it now, and let it fan the spark of virtue that yet remains within thee. Choose thee now to save those that cannot save themselves, to reclaim thy angelic mantle, and to set Elturel free!

(OOC: I would have liked to spend more time writing this speech, but I'm already gonna be late for work, haha! Not sure what you'd like, but it seems appropriate to make a Persuasion check. I'm using Inspiration for advantage, and adding Guidance from Choir. You might notice that Ordrag is using some of Zariel's own words that he heard her speak in the dream.)

Persuasion: 30

(Oh, that's a nat20 !!)

(Didn't realise Astalda had posted first. Modified my post a little.)

((The Persuasion was necessary and DC was 25 with possible +/- for circumstances :) It was meant to be!))

Polly's lack of comedic streak rivaled only Zariel's complete lack of sense of humor. But the archdevil did not have time to act on potential misunderstanding. She still snapped at Choir "Elturel is exactly where it belongs with every traitor put to good use." But even as she spoke her eyes were fixed on the sword that Ordrag brought so close to her, that holy light almost overcome the hell fire in her eyes. "Your soul shines bright,” answered archdevil to the Ordrag's call. “Brighter even than the beacon above Elturel. You do know what sacrifice means. And so do I. All I have ever done was for greater good! Do not tell me it was for nothing!"

She was talking, but her voice already betrayed the storm of feelings the sword, that seemed to shine even brighter, and the words raised in that burned wounded soul - it was trembling.

"Zariel," Lulu in her golden mammoth form stepped closer. "I returned for you. I know you are still there, that place - it could not overwhelm you, nothing ever could! You saved me, you saved that sword too at the worst hour. What you are doing to the city - this is not you! Two wrongs never make things right, you taught me this yourself long ago. Come back, Zariel! Stop that madness and come back, friend!"

Limping Mooncolor stepped forward too lowering to one knee, adding his soft celestial glow to light of the heavens coming from the sword. The weapon slipped from Ordrag's grasp and levitated to hover before the fallen angel. Zariel’s trembling hand reached toward the gleaming hilt of her sword. Her fingers brushed against it, and she grimaced as radiant light seared her flesh.

As her grip tightened, she gasped in pain,  but then then spoke through tears of confusion, sorrow, and dawning joy. “I, Zariel, supplicate myself before the holy light of justice. If it should accept me, I vow to take up this blade once more in its service.”

The archdevil’s words hanged in the air for a silent moment, and she glanced upward in agonized uncertainty. But the moment passed and  a brilliant wash of radiant light bathed Zariel in. Her fiery halo disappeared, and feathers burst from her leathery wings. All of her diabolical features vanished as her angelic form was restored. At the sight of this blessed transformation, Lulu gasped in delight — the angel - her angel! - returned.

"If I am to save the city - I have to do it now! It will return to the surface instantly - are you willing to go there with me? I will not be able to return for you later."

Mooncolor, that rarely spoke up, answered first - he was ready to go and could take three people with him. Lulu with her wings did not need it and was going to stay with Zariel no matte what. But others had to decide here and now.