Arcturus just barely manages to resist the oppressive mist. He wobbles a little on his feet, but is able to take stock of the situation around him, searching for something to use as a weapon. There are no weapons on the ship that he can see, which makes him think that the shield was the only line of defense this ship had.

The fiend chuckles as the paladin dodges out of the way then sends divine power at it. It groans as Srikandi sinks her blades into the foul thing, and rumbles "You like to play, don't you? Well, how about we have some fun?!"

A roaring sound fills the air as a long, eldritch tentacle slithers forth from the void mouth of the thing and strikes at the ranger, intending to wrap her like the coiling tactics of a constrictor snake. Dark spines dot the tentacle, scything into flesh as it attacks her. The fleshy part of the tentacle is exposed, a thick, pulsing section that could easily be cut. 

Meanwhile, the orb above head seems to be growing, as the demon glances upward, seeming to be checking on the orb as it continues writhing in the air. Voices begin chanting again, compelling the fiendish object to become more vile.

The Demon makes a dark tentacle attack against Srikandi:

• Dark Tentacle Attack: Melee Spell Attack; 19 to hit. Hit: 13 Slashing damage and she must make a DC 14 DEX or STR (her choice) save or be restrained. The tentacle has an AC of 9, and can be cut by dealing 5 or more slashing damage to it.

The Demon also rolls a DC 14 CHA check to empower its Demonic Orb: 22. On a success:

The Orb grows and pulses bigger, until it is the size of a household pet such as a large cat or small dog. A scaled, green claw reaches through and gropes around in empty air.

Otherwise, the orb does nothing except pulse. 

Here's a tip for this fight: if you're not melee fighting the Demon, make your Mist when you make your action post!

It is now Kaelos's turn! Let's see what you've got!

After seeing that Despario was alive, Kaelos shared a concerned glance with Arcturus.  It was time to take the offensive!

Sheathed in his protective ward, the bard drew his enchanted longsword and closed in for melee, swinging his blade at the crimson demon with a shout that was equal parts courageous and defiant.

Longsword attack: 14, 5 slashing damage.
Concentrating on Blade Ward.

Srikandi Dex Save- 14

(( Arcturus WIS Save 4 ))

Kaelos charges forward, the courage of the heroes he has heard so much about filling him as he valiantly swings his swords with all his might. The Crimson Demon growls, and manages to throw up an arm, forming a shield of red energy as the weapon just abrely misses, clanging off the protective armor with a spray of red sparks. The Demon chuckles "Your naivety will be your downfall!" It proclaims

It raises its hands, and two weapons of energy appears in its outstretched hands. One is an axe, composed of the essence of the void, which writhes in the hands of the demon as if a hound wanting to be let loose after a rabbit. The other is a long, curling whip of fire, the tip the head of a flaming snake that hisses at the bard hungrily

The Demon swings its weapons, letting loose their fury. He cracks the whip twice, and the serpent screams a bloodcurdling spring as its fangs hurdle towards Kaelos, intent on filling him with the fires of death. The demon then swings it around at Darius, attempting to slice him as if the whip was a sword.

As the fiend makes two vicious attacks, it lets loose the powers of the axe. It rears back, and sends the necrotic blade flying through the air, sailing towards Arcturus, though his prone state and the tactical position he took make the ends of hitting unlikely, as Arcturus can tell. Still it is a deadly axe. 

The axe soars through the air, and once it hits something solid (either Arcturus or the tower beside him) it falls to the ground, shimmering and glimmering like obsidian, its power apparent. This is a useful tool, and maybe you could use it to your advantage.

The demons make two hellfire whip attacks and one void tomahawk attack

• Hellfire Whip Attack #1 VS. Kaelos: Melee Weapon Attack; 21 to hit. Hit: 7 Slashing plus 5 Fire damage
• Hellfire Whip Attack #1 VS. Darius: Melee Weapon Attack; 14 to hit. Hit: 5 Slashing plus 3 Fire damage
• Void Tomahawk Attack VS. Arcturus: Melee Weapon Attack; 10 to hit. Hit: 7 Slashing plus 4 Necrotic damage.

If anyone wants to use the Tomahawk, it functions as a +1 handaxe that deals an additional 1d6 necrotic damage

It then tries to empower its demonic orb: CHA check (DC14): 21; If it succeeds:

The orb enlarges, and a whole arm appears. It is mottled and scaled, green and purple in color. Thickly muscled, you realize that this arm must belong to some very vicious fiend. No mere imp has that big of a claw. 

Darius, Arcturus, York, and Srikandi, Let's see what you've got! (York and Arcturus will need to make their Mist saves at the end of their turn)

York stands, trying to shake off the pain in his head.

He will cast Magic Missile and upcast it to 2nd level, sending three at the Demon and one at the orb/claw.

Demon: 6 force damage [rolled 3 1's]

Orb: 5 force damage

Wis Save: 17

Srikandi continued slashing at the creature, " Your manners leave much to be desired!"

Stay in Melee.

Maintain Hunters Mark.

Attack: 9 Damage: 10 + HM 5 Force.

TWF/Nick Attack: 10 Damage: 4 + HM 4 Force.

Once again getting up from his prone position, Arcturus finds himself unable to do little more than regain his wits as best he can as the Red Mist continues to assail his mind. He tries to take concealment before lobbing an arrow at the enemy. After, he moves to get within melee range if he can... better do die fighting than on the ground. 

(( Bonus Action (Cunning Action) : Hide 26 (w/advantage from Skulker feat) ))
(( Action: Sneak Attack (Short Bow): 19 to hit, 15 piercing damage ))
(( Movement: Stand up from Prone, 15' towards enemy ))
(( Wisdom Save: 12 ))

Darius in a slow motion sequence perfectly dodges and yells at the hellfire whip as it passes by his chain armored torso, "Back to the pit Demon!"

Action attack 18 to hit.  If this hits *.  If it misses **.

* Damage 12 slashing + 4 radiant +7 (or 9 if opponent is a fiend), radiant from divine smite.  As a bonus action after using divine smite Darius will channel activating inspiring smite...
6 temp HP to Srikandi and 5 temp HP to himself.

** If the attack misses, bonus action shield of faith Srikandi +2 bonus to AC

(Just need to know if a hit or miss)

Kaelos grimaced as the hellfire whip struck him, but his blade ward did not fail!

Con save 16 vs DC10

York sends out flashing darts of arcane power that flies through the air, unerring and deadly. They strike their targets, and the orb withers as the projectile destroys some of its power, growing smaller and weaker.

Srikandi continues her vicious assault, trying to end this fight before her luck runs out and the powerful attacks of this demon kill her. But her focus is thrown off by the rush of her companions, who crowd around the demon. Her blades miss, the creature diving out of the way.

And right into Arcturus’s arrow, which flies through the air as the rogue manages to shake off the debilitating mist. The deadly arrow strikes out, causing the creature to shriek, and twist, but it does not fall.

As the creature cries out, Darius feels divine power flow through him. This thing is an abomination, and one he cannot stand for. He raises his blade, and strikes down, his weapon gleaming like an arc of sunlight cutting through the shadows. His blade slices through flesh, and and the creature roars in pain.

At this onslaught, the creature finally acknowledges the power of each of you. “You are stronger than anticipated. You fight for your friends. But you should fight for my lord instead” he growls, focusing its power on York.

A dark tendril slices through the air, piercing his forehead, though it deals no damage, being ephemeral. It fills his mind with anger and rage, the desire to serve the master, the desire to harm those who truly steal from the glory of your master.

I need York to make a WIS save (DC 14). On a success, he’s fine. On a fail, he is corrupted until the end of his next turn. While corrupted, he treats everyone as an enemy, and he must use his actions to damage the biggest threat he can perceive, and trying to damage multiple creatures if he can.

Kaelos, your turn!

[Oh Boy...]

Wisdom Save: 15

Kaelos brandished his sword at the demon.  "Nice try, beast."  He admonished, weaving some of his lyrical magic into the words of the insult.

"However, I think you'll find the Shield Wardens to be made of sterner stuff!"  Here, the bard threw an encouraging glance at the ranger, bolstering Srikandi.

...

Action: Vicious Mockery vs crimson demon; Wis save [DC14] or take 6 psychic damage and have disadvantage on next attack roll it makes before end of its next turn.

Bonus Action: Bardic Inspiration on Srikandi
EDIT — Forgot to add: Concentrating on Blade Ward

Trying his best to ignore the mental agony imposed by the mist, Arcturus realizes his ranged attacks may be a tactical advantage for his allies. He tries his best to concentrate and fire his bow again, moving no closer to the enemy. 

(( Bonus Action (Cunning Action) : Hide 27 (w/advantage from Skulker feat) ))
(( Action: Sneak Attack (Short Bow): 26 to hit, 12 piercing damage (used one Lucky to gain Advantage, one remains) ))
(( Movement: None ))
(( Wisdom Save: 12 ))

Kaelos raises his sword, sending it high into the air. It gleams in the light shed by Darius, and the creature shrinks back as he unleashes a scathing exhortation, both filling his foe with dread and empowering his companion!

While the demon does not suffer wounds, the regeneration it has to close the wounds slows down as energy within its shifting form is destroyed. The core of its being begins to dim, but it still keeps a cohesive form, fighting on with a rage,

The creature braces itself, then summons the glimmering axe back to its hand. The whip disappears, and it grasps the void weapon in its hand, as the axe grows in size until it is a weapon of truly massive proportions, which it swings about its head before bringing it down in a mighty crash that sends you flying.

WIS save Vs. Vicious Mockery: 12

The Demon makes a Crashing Void Strike attack.

Crashing Void Strike Vs. Srikandi, Darius, and Kaelos: Melee Weapon Attack; 16 to hit. Hit: 15 Necrotic damage and you are pushed back 10ft.

Darius, York, Srikandi, GO! (I'll take post #55 as Arcturus's turn this round and narrate the results when everyone else has gone)

*Sap from long sword hit.  
Either way-

Action attack 22 to hit  
Bonus action Inspiring smite.  
Damage 12 slashing 19 radiant.  
temp hp 19 to Darius.

Movement if necessary to get into melee.

”Time for your destruction!”

Srikandi grimaced as she fought on, " Your tough, I'll give you that."

Stay in Melee.

Maintain Hunters Mark.

Attack: 13 Damage: 9 + HM 6 Force.

TWF/Nick Attack: 26 Damage: 4 + HM 6 Force.

York will aim for the Orb again, the sound of a bell assaults it. Wis 15 save or 11 necrotic damage.

York's Wis Save: 24 (nat 20)

Good point Hunter, I’ll keep that in mind in the future!

Toll the Dead WIS save: 7

The demon swings its weapon, but the heroes stand steadfast. They ready themselves, preparing to smite this foul being once and for all!

Ringing sounds fill the air as York and Srikandi attack in tandem, blades and spells clashing with power, a mighty burst echoing forth. York fills the air with the sound of grim bells that signal doom, the withering power of time consuming the demon as Srikandi plunges her blades deep into the flesh of the foul thing.

Then, Arcturus sends forth a single arrow. One humble arrow, that flies through the air, before embedding itself deep within the roaring void mouth of the demon. The creature falls to its knees, and looks up at you as a blinding light steps forward

Darius raises his blade, and like a brand of light slices it at the foul beast. It is a mighty hit, the divine power that explodes upon the flesh of the demon once again fills the air with ringing. The head of the demon falls to deck, still moving in agony

The fiend screeches, and begins to dissolve, its power fading. Thankfully, the orb ahead also falls out of the sky, solidifying then shattering on the ground as if made of glass. The arm of the monster inside is severed, flipping around on the deck before going still and crumbling into ash.

the red aura that permeated the fight now disappears, and you all feel the cold most pressing around you. You need to revive your captain and get the shield back up, quickly, or the mist will drain you of all life.

You look around, and notice some sort of control device atop the tower. Maybe that could fix the shield? Or maybe you could break it further?

We are still in initiative, but the demon is now removed. the new order is

• Melees (Kaelos, Darius, and Srikandi)
• Ranged (Arcturus, and York)

Darius, Kaelos, and York, due to your extra-planar knowledge, you know that that the average hero of your caliber could survive the mists for about a minute. York and Arcturus have already been subject to about half a minute of mist, while the rest of you just started experiencing the mist. Work quickly!

Melees, it’s your go! What do you do?