(@Khazeah: I haven't heard from you yet on that {DC 15} end-of-turn INT saving throw so, to keep things moving, I'll go ahead and roll that for you here: 13.)

Unable to continue fighting for now, Shai wobbles a bit. Her dizziness persists even after a few seconds pass. 

Seeing her friend in need, Quadere prioritizes her well-being and taps her blessings once more. The curative magic comes in the shape of kind words this time, and Ven and Kandalou hear them loud and clear as well. (Shai, Ven, and Kandalou each heal for 7 HP.)

Not yet done communing with Oghma, Quadere then calls on her sacred flame to descend on her enemies once more.

-> Mind Flayer's DEX saving throw {DC 16}: 10

The illithid feels one of its tentacles burn righteously. Around this time, the duergar warlord regains his faculties. (Mind Flayer takes 13 radiant damage; Duergar Warlord is no longer stunned.)

Kalak sees the ilithid behind the duergar and sends four burning lances its way (Upcast to 3rd level from J-17). "We will not suffer you!"

Ray 1 Attack: 23 Damage: 10

Ray 2 Attack: 26 Damage: 5

Ray 3 Attack: 13 Damage: 6

Ray 4 Attack: 16 Damage: 8

Ven is the first to react aggressively to the mind flayer. He advises his fellow hunters to spread out and sprints eastward putting in practice his advice to the rest of the group. While moving, he begins to weave his next spell and, in fact, is so concentrated in the process that he doesn't even notice that the warlord has now re-tightened his grip around his hammer:

-> Warlord Hammer vs. Ven: Attack: 27 Damage: 44 magical bludgeoning and psychic.

Both his back and his head now hurting, Ven manages to hang on and finish his spell. (Ven takes 17 magical bludgeoning and psychic damage.)

-> Mind Flayer's CON saving throw {DC 15}: 8

The mind flayer braces itself for the loud arcane impact, diminishing its effects. (Mind Flayer takes 15 thunder damage.)

The skillful bard still finds a moment to add a couple of encouraging words to the end of his magic-laced incantations. Even in her dazed state, Shai hears them and knows that the fight is not over. (Shai gains a d8 Bardic Inspiration to use at her discretion.)

Not wanting the warlord to be a nuisance any further, Vishnu resists her urge to attack the illithid and puts an end to the duergar's fight with a well placed arrow to the knee. The shock is enough to force the dwarf unconscious. (Duergar Warlord takes 9 magical piercing damage and is DEFEATED.)

(@Superbad: In the interest of keeping things moving, I'll go ahead and roll your {DC 15} end-of-turn INT saving throw as well. Yours includes Bless. Here it is: 9.)

Kandalou is completely unaware of what is happening around him at the moment. His headache is not abating for now.

Kalak takes a few steps west and launches four burning lances at the mind flayer. The first of them hits a rocky wall, but the other three are as accurate as they come. The illithid uses his hands to snuff some flames on his robes. (Mind Flayer takes 22 fire damage.) 

(Mind Flayer goes next.)

DM's Screen:

The illithid steps out from behind the now-slumped-over warlord and advances on Shai.

-> Tentacles vs. Shai: Attack: 13 Damage: 17 psychic.

By a stroke of luck, a still dizzy Shai wobbles away from the attempt on her cranium just as the mind flayer stretches out its tentacles. Fearing retaliation from its target's companions, the brain eater retreats around the corner.

DM's Screen:

END OF INITIATIVE ROUND 4

PORTAL APPROACH - INITIATIVE ROUND 5

• 23. Shai (24/66) - stunned, incapacitated, Bardic Inspiration (d8);
• 20. Quadere (66/66) - concentrating on Bless (counter: 6);
• 18. Ven (32/57) - Bless;
• 15. Vishnu (66/66) - Bless;
• 15. Duergar Stone Guards  
• 12. Kandalou (60/94) - stunned, incapacitated, Bless;
• 08. Kalak (38/38) - 
• 03. Duergar Warlord (defeated but alive) - unconscious, incapacitated, prone, Enlarged;
• 03. Mind Flayer - 

MAP:

 

NOTES: It's dim light everywhere in this cave. Unless you have a way to counteract it, your perception checks and search actions are done at disadvantage. It does not affect attack rolls.

(Shai, Quadere, Ven, Vishnu, Kandalou, and Kalak may all take their turns.)

(@Khazeah & @Superbad: You are still stunned. Please roll a {DC 15} INT saving throw at the end of your turn.)

Shai's eye go wide in terror as her worst nightmare approaches with her powerless to act.....and thanks the gods as the Illithids tentacles fail to gain purchase on her face....

Determined to break the hold the beast has on her mind she focusses inwards, but cannot free herself.....

Save: 2 plus Bardic Inspiration 4 Edit: If use of Bardic Inspiration is a choice after the roll then I would have held it to next round....

.....it does not get much worse than that!

I assume that Shai cannot take any action, but if she could she would use the opportunity to use Wholeness of Body (27 HP), but I am assuming that in a Stunned state she cannot.....

Ven doesn't want his prey to get away. He rushes forward, enough to see the creature and disappears in a silvery mist (misty step) before reappearing behind the mind-flayer with his rapier in hand (R14).

"Not so fast"
He then delivers two quick attacks.

Rapier1: 23, piercing damage 6
Rapier2: 9, piercing damage 10

Vishnu curses vigorously once more, then sends a stream of arrows off at the mind flayer.

Attack: 22(3 Damage: 8 magical piercing

Sneak Attack: 12

She’ll move back into the shadows and hide as a bonus action, if possible afterwords.  Moving to wherever she could be unseen.

Stealth: 14

Quadere, concerned about what the Illthid is planning, decides to dash to R21 and as soon as the mind flayer is in sight, calls to Oghma “Bidh mi ag iarraidh an armachd spioradail agad!” and for her ba casts Spiritual Weapon, causing a glowing longsword to appear by the creature and attack it.

Attack: 19 Damage: 11

(@MuggsHill: Go ahead and keep you Bardic Inspiration die. You wouldn't have used it after rolling a crit fail. Also, I'm afraid that Wholeness of Body requires an action, which you cannot perform at the moment...)

Shai continues trying to get a hold of herself but can't quite escape the mind flayer's own hold on her mind.

Quadere goes on the offensive again and dashes forward. Upon reestablishing visual contact with the mind flayer, the cleric summons her spiritual longsword, which appears on the other side of the enemy {S-14} and swings. The move catches the illithid by surprise, cutting its back. (Mind Flayer takes 7 force damage.)

Ven cuts off the mind flayer's escape rout by suddenly appearing behind it. This quick thinking allows the bard to land a solid first strike on the illithid, but the creature manages to avoid a second. (Mind Flayer takes 6 piercing damage.)

Vishnu also moves to get a better angle on the illithid {Q-23}, releasing an arrow as soon as she sees her target. Now facing the recently teleported Ven, the mind flayer never sees the the arrow coming for it. The projectile dives into the back of its neck and reappears on the front. Ven winces as the brain eater falls to the ground. (Mind Flayer takes 23 magical piercing damage and is DEFEATED.)

Vishnu then gets as close to the nearby wall as possible, preferring to let a bit of shadow cover her before being reassured that the battlefield is now safe. The rest of the rrakkma looks about, expecting another threat to jump out of a corner, but none does. After about 45 to 50 seconds, both Kandalou and Shai regain their faculties. A few moments later, Quadere feels her summoned blessings also fade (Bless and Spiritual Weapon.) The duergar warlord is the only enemy left alive, though he is not responsive at the moment. His magically-fueled enlargement also runs its course, as do those of the now dead duergar in his charge.

YOU ARE NOW OUT OF INITIATIVE. YOU MAY DO AS YOU WILL.

In an effort to stop the escaping ilithid, Kalak uses his psionic ability to Kalak send himself (psionic ability racial feature) to the other side of the large bodies and launches a fire bolt at the Giths' eternal foe.

"You will not be going anywhere, encende!"

Fire bolt vs ilithid Attack: 11 Damage: 11

(Nevermind it's already dead)

"Well, if that is not confirmation that we are on the right path, I don't know what is," Kalak says as he sneers at the mind flayer's corpse

Vishnu comes out of the shadows, and calmly spits on the dead mind flayers.  She says nothing, just begins collecting her arrows.

Quadere speaks to the group: “With that done we must now decide, are we ready to proceed? I know I used up more divine energy than I anticipated so quickly. That being said, shall I provide more healing for those of you injured?”

As the Mind Flayer falls, Shai finally seems to come to her senses, and her face seems a little haunted as she begins to feel the magnitude of the task, looking to the others and observing "This is a mighty task we are attempting........may the gods grant us strength".  With that she briefly takes the time to close her eyes and gather her Ki, feeling its healing warmth repairing the damage from the Warlords blows (heals 27HP) then stands a d moves to the Warlord...kicking him onto his front she will remove his armor and weapons and anything else she can find before grabbing her rope out to bind him extra securely, mindful the he may have the power to enlarge, so ensures that both arms and legs are secured such that enlarging will will be restricted (causing immense pain if he tries).

Once secure...she calls out "Ven.....he's all yours..." knowing that interrogation is right up his alley.  If and when Ven does start questioning the dwarf though, Shai will use Detect Thoughts to assist in discovering the truth more directly from his mind.

If no one else does, Shai will also go and search the Illithid methodically after securing the prisoner, anger apparent as she recalls how powerless she was.

Quote from MayhemIsEverywhere >>

Quadere speaks to the group: “With that done we must now decide, are we ready to proceed? I know I used up more divine energy than I anticipated so quickly. That being said, shall I provide more healing for those of you injured?”

 With Wellness of Body - Shai is back up to 51/66 - so could use topping up, or we could take a Short Rest and use some HD to recover rather than use more spell slots, which will also reset some other skills.

“I agree, let’s have a short rest. I can play some music I learned from the fey realm. It has a healing power” (+1d8 hp)

Ven looks at the now dead illithid. “First let’s see if you got anything on you”, says Ven, while checking the possessions of their nemesis.

@Ven: please roll an investigation check with advantage.

@Shai: please roll an investigation check and a separate DEX check.

((I apologize for my brief absence! Work, man. Work.))

Kandalou will let out a sigh of relief after the stun goes away and mutters, "You'd figure I'd get used to that by now." He will look around and take in his surroundings and see the Warlord bound, ready for interrogation. "Right up my alley as well, Shai." he says in his gruff monotone voice.

He will sheathe his sword and walk calmly to the warlord and Ven to assist with the interrogation. ((Believe that gives his roll advantage, yes?))