Choir moves to the center of the group, calling Mooncolor to come in closer, and casts Magic Circle against Fiends. That should make it harder for the demons to maul the party.

((Inconsequential since fog dispelled but would Polly’s Devil’s Sight have allowed her to see anyway?  Guessing no but unsure...))

     OwImp will fly away.  A bit.  The familiar flies higher but stays within range of seeing and shouting out positions.  It’s dangerous, though! He mentally complains to Polly but she sternly instructs him to suck it up and do as he’s told.  

     Polly will also takes to the air.  Stretching her wings in battle for the first time, Polly raises herself up 30 feet up into the air (full flight speed) and casts Darkness upon herself to make herself a hovering ball of no light.  

As the demons approach, Ordrag draws both handaxes. After the fog clears and he sees the attackers at the end of the barge, the half-orc throws both at the glabrezu.

(Disadvantage for long-range)
Handaxe #1: 20 Damage: 5 non-magical slashing
Handaxe #2: 9 Damage: 8 non-magical slashing (Savage Attacker re-roll - keep highest: 10)

Then free action to draw both longsword and warhammer

Adray crouched, ready to rush the Glabrezu when Chior's spell went off and a circle spread out.  Regardless he wanted to run, to rush and attack, something within him screamed for it.  By sheer force of will he remained within the circle, he trusted Chior's judgement here.  Instead he found himself balling his hand into a fist, drawing back and punching straight at the Glabrezu.  He roared out like a dragon and from his outstretched punch a sparkling bolt of radiant dragon fire flew out.

Action: Guiding Bolt at the Glabrezu

Attack: 22 Damage: 15

(Devil’s Sight is for Darkness (I am aware of the most bizarre RAW in the book, but we will treat Fog and "heavy obscured" conditions differently from Darkness). Glabrezu has a True sight, for him invisibility of any kind does not exist)

Fran seemed to surprise the smaller demon with her attack - at least one hit, letting the demons know the prey is not that easy to catch.

Glowing runes appeared on the dirty deck around the party (blue circle), protecting shimmering light to the evil sky,  and Polly flew above it, covering herself with the cloud of darkness. Very serious Lulu jumped inside too, adding her glowing light to the Choir's circle.

Demons did not need a special invitation by this point, but Ordrag and Adray added to attackers anger - even demonic hide did not protect Glabrezu from the spell. Demons were about to rush the party when Mooncolor thanked Choir - and suddenly in a blink of an eye appeared in front of the huge monster.

11 (G - 18 )

Whatever Mooncolor tried to do seemed to fail, but he added his hooves to the attack: Attack: 9 Damage: 12 and did his best to block the path of the demons.

Round 2

Unicorn was simply on the way and irritating enough to be attacked. Both small demons jumped on Mooncolor in unison, both attacked - and then both disappeared. But Glabrezu did not go anywhere, taking his time to destroy this celestial:

H1 vs MC: Attack: 15 Damage: 15

H2 vs MC: Attack: 26 Damage: 10

Gb vs MC: Attack: 28 Damage: 19

                   Attack: 20 Damage: 21

The demon let the party to see his pincers grabbing Unicorn before covering the area around with Darkness.

((Result: Hydroloths - nowhere to be seen, Glabrezu and Mooncolor are hidden with Darkness, Polly is above the deck also in the sphere of darkness))

Everyone

     Polly flies her full 30 feet movement towards Glabrezu and Mooncolor, gaining maybe another 5-10 feet of altitude while she does.  During this, Polly places her left hand against her heart in the shape of a first, then thrusts the hand out towards Glabrezu, fingers splaying at the last moment as she says "Boed i'r chwyth hwn eich taro'n farw!" and unleashes two bolts of Eldritch Blast at the demon.

First Eldritch Blast Attack: 12 Damage: 11

Second Eldritch Blast Attack: 8 Damage: 6

     Owimp still circling the battle, watching for more entrants heading towards the melee, shouting updates, cursing creatively for little reason but otherwise trying to stay as far away as possible while still being effective.

Sorry sorry sorry, I said Adray did guiding bolt but didn't say the next attack had advantage.  Sorry I assumed to much.  Maybe mooncolor hit with something?

((Well I would think next attack would be by initiative order, not posting order?))

So we can freely leave and re-enter the circle?

(As I understand this spell - yes.)

Fran would visibly clench her teeth as she’d witness Mooncolour step into the fray and get bombarded with attacks. Then they vanished and their unicorn was absorbed in darkness. Now she couldn’t see shit. A rumble in her throat would start as she’d scramble her thoughts for ideas. The artificer would then tear a cloth soaked in strange liquid into three strips. “Dun ask...” She’d mutter, skipping over explaining the ingredients and tie a strip on Adray, Ordrag and Choir.

Casting Aid on Adray, Ordrag and Choir 5 temp HP

Fran couldn’t just stand back and let Mooncolour be bait.

(OOC: Seems foolish to leave the magic circle on account of the unicorn. I think I need to get Ordrag a ranged weapon. Will probably regret this...)

(No, I changed my mind. These guys are formidable enough without putting myself at such a disadvantage)

Ordrag moves to the edge of the circle and Readies an attack in case any demons appear beside him

Adray moves forward to the edge of the top deck.  He puts his hands upon the rail and shouts at the Glabrezu, "I thought Glabrezu's were tough, but I guess they hide like cockroaches in the dark.  Pitiful."  He props himself up with an intimidating stance.

Intimidation: 17

action: Blade ward cantrip 

None of Choir's spells can dispel the darkness, or heal an unseen unicorn in the distance. She throws one of her javelins into the darkness, hoping to hit a demon and not Mooncolor...for what little damage the non-magical missile will do.

Attack: 14 Damage: 7

((Aid is accepted - did not add it to the CS, but will count when needed))

Brutus disappeared in the dark cloud and so did the two bolts of energy leaving one black sphere  to be swallowed by another. Choir's javelin flew though the globe, according to Owimp not hurting anyone inside.

Owimp reported that Mooncolor is still alive, if hurt (Polly could see it by herself, though). He even tried to do something 8 that made him glow for a moment. (Demon did not seemed to be affected by it)

It is unclear whether Glabrezu did not hear Adray or did not understand, but it certainly did not react. However, there was someone who did - one of the Hydroloths. He tried to claw Adray:

H2 vs Adray: Attack: 15 Damage: 14

while another one mumbled and obvious spell, sounding very familiar with the words Glabrezu used to get rid of the fog and pointed, as it seemed, at the runes of magic circle. To his surprise - nothing happened. The demon looked at Lulu and small hollyphant whispered "just try it!", not moving herself (and her aura) away though.

Mooncolor was still blocking the path for Glabrezu and still tried to fight it:

Attack: 11 Damage: 7

Attack: 19 Damage: 6

and it seemed Glabrezu did not mind fighting celestial either - he even enjoyed it:

Gb vs MC: Attack: 21 Damage: 16

                   Attack: 17 Damage: 14

But then it cast something on itself without visible result at first.

Everyone!

Adray turns to the attacking demon, H2, and lays into him with a beastial furry.  As his attacks fly he feels his ki flowing and his fists turn greenish.  

Attacking H2 with flurry of blows, all acidic damage

Attack: 18 Damage: 5

Bonus action flurry

Attack: 22 Damage: 5

Attack: 25 Damage: 8

 Should have been one more attack under the normal attack.

Attack: 26 Damage: 7

First hit will try stunning fist, save dc 14 con or be stunned.  If stun is effective here are three attacks for advantage. 9, 27, 18

     Polly more or less repeats herself... Placing her left hand against her heart in the shape of a first, then thrusting the hand out towards Glabrezu, fingers splaying at the last moment as she repeats "Boed i'r chwyth hwn eich taro'n farw!" and unleashes two bolts of Eldritch Blast at the demon. She continues flying towards the demon’s location, but upwards a bit, the goal to be hovering 30-40 feet above the thing.

Attack: 11 Damage: 12

Attack: 19 Damage: 9

OwImp, uncovered by darkness and (I think) closer will also fly to above the demon and scream out “It’s right down there!” while pointing below himself.  “Aim below me you stupid mortals!”

With the frog-demons now visible and nearby, Ordrag joins the battle. Almost without thinking he yells: Avast, you bilge rats! This feels a lot more like his days on the Grinning Oyster fighting pirates, except the pirates are frog-demons and his shipmates are all sorts of strange folk and the scenery is red and apocalyptic and the water is blood. But other than those things, very similar.  

Action: Attack/Longsword.
Attack#1: 27 Damage: 10 slashing.
Attack#2: 12 Damage: 10 slashing.

Bonus Action: TWF/Warhammer.
Attack: 24 Damage: 13 bludgeoning.

Action Surge: Attack/Longsword. 
Attack#1: 19 Damage: 13 slashing.
Attack#2: 25 Damage: 7 slashing. Savage Attacker reroll (use highest): 24

All attacks against H2 (same target as Adray).

And sorry I already added the Aid to my CS.

Fran would focus on a target she could see, releasing two more bolts.

Attack 1: 20 Damage: 8 
Attack 2: 28 Damage: 12 

Choir wishes she could move the magic circle with her: but at least she can leave it to defend those with ranged weapons. She is unable to help the unicorn, still hidden in darkness, but she can help Adray, at least!

Choir runs out of the protective circle to stand south of Adray, where her spell can hit both demons:

Thunderwave, level2: H1 and H2 take 15 Thunder damage, Con save DC16 for half damage, or be pushed back 10 feet.