So... Yes to the Pax's Hex and the boots being active before the barrier drops. But that's all! No more! That's why I asked you all to declare your preparations BEFORE it happened. Sheesh! And since you feel I was being generous not to injure Pax for running into a wall at high speed, I'll simply penalise her instead by denying her any other action for this round. Give and take. She was shocked and disoriented enough that it took those precious moments to get her bearings, but she still had time for misty step.

By the way, I also recently realised that Hex only affects ability checks, not saving throws. Or did I mention that already? Just tossing it out there again in case it comes up.

Also, I've resolved my confusion with Sekkar's actions - basically my not knowing my own maps and misunderstanding the wording of Zephyr Strike (in fact I was too generous!) but all solved by jiggling the timing.

Also, just to avoid confusion (or more likely, to commence the rules debate) the invisible wall that Pax just met was in fact a Wall of Force. These characters have already encountered one, so I don't mind them concluding that the same effect is occurring, and that it prevents physical passage (besides teleporting) and also prevents spells being cast through it. The fact that the brain (and Pax and Sekkar) are still visible doesn't change the fact that they have full cover and cannot be targeted by spells. If you have any specifics you want to discuss, then go ahead (so I can shoot you down mercilessly).

As Pax appears beside the brain clutching her rod of tentacles, the brain ironicaly lashes out at Pax with one of its own tentacles.

Tentacle Attack: 14 Damage: 27 bludgeoning, and on a hit, Pax is grappled

Ah, Kirin just posted before I could post my spiel about the Wall. Yes, Kirin would probably know about the wall and that Bigby's Hand would likely fizzle. Sorry I wasn't fast enough.

"As I do not have the means to bypass this. Mr Sekkar? Could you attempt to smack it a lot to stop it's ability to keep the wall up?" Kirin attemps to shout through the wall as he ranks his brain for ideas but settling on just being ready to Dodge

Kirin racks his brain on ways to wrack the brain. He knows the creature must maintain some level of mental focus to maintain the magic wall. And such focus can only be broken by suffering trauma, or from casting another such spell. If only there was some way to force the creature to cast another spell requiring the same degree of concentration... If only...

Meanwhile, another tentacles whips around at Pax...

Tentacle Attack: 10 Damage: 22 bludgeoning, and on a hit Pax is grappled

...immediately after which the elder brain sends out a shock of psychic energy, similar to but more powerful than that executed by the mind flayers in the corridor. Thankfully for the others, the wall seems to block the effect, and only Sekkar and Pax are affected...

Pax and Sekkar have all the fun! Please roll a DC 18 Intelligence saving throw or take 36 psychic damage and be stunned for 1 minute.

Derendil and Kaylyn can both go next.

Round 1
Rhayna (26) Shillelagh (1/10), Symbiotic Entity (8tHP), Bless (conc 1/10)
Sekkar (22) blessed, Zephyr Strike (conc 2/10)
LAIR ACTION (20)
Pax (17) blessed, Armor of Agathys (15tHP), Arcane Ward (6HP), Hex (conc 2/10)
Kirin (16) blessed, Mage Armor
ELDER BRAIN (11)
Derendil (10) blessed
Kaylyn (8) blessed

Another tentacle slams across her heavy armor.  Then Pax feels the sudden psychic pressure and her face screws up as she tries to focus on letting it wash past her.

Int save vs DC 18 : 21 

As the immense energy washes through her psyche, she sees herself in her minds eye, standing amidst a maelstrom. She draws the shadows up from around her mentally and lets the winds wail about her...until they blow themselves out.  She hadn't realized she'd had her eyes clenched shut until the energy was gone and she opened them, blinking in the darkness...tears from her effort trickle down her cheeks.  But....she has withstood the storm. 

Sekkar Int Save: 14 + 4

Sekkar did not know it for sure but could feel that blessed light of Rhayna repelling the brain's attempts to get into his mind. Fortunately, for the moment this thrice the monster was playing with Pax. If the lizardman could hear Kirin, he did not indicate it, but his actions were of those the wizard wanted - smash until the monstrosity is no more.

Meanwhile, Derendil was desperately scratching the invisible wall, slowly remembering "seen" such things before - not often, you do not need such powerful magic to keep slaves in line, but that happened too. Hopeless and pointless, dividing and leaving no escape to the trapped. Derendil raised his head and howled a cry of desperation.

Sooooo... Just to add some more fun to the wall of force, would Kaylyn see that the brain cast the spell and so as a Reaction cast Counterspell?  (Gotta love play by play)

If so DC is 15 15

As a Bonus Action she gives Sekkar Bardic Inspiration 1d6 "May your sword never dull!"

Just replying here in addition to my lengthy post in the message board...

No, Kaylyn wouldn't see the spell being cast in this case. Also Bardic Inspiration doesn't appear to inspire Sekkar, as he seems unable to hear Kaylyn (and you don't expend that use of it). Kaylyn and the others on her side of the wall likewise cannot hear the sound of battle they would expect from Sekkar and Pax. 

((Well it was worth a shot, I wasn't sure if that would work as I figured it was a lair action.))

Kaylyn looks to Kirin and Rhayna, "I can't leave them to fight alone, I'm going in to help." Determining where the wall is Kaylyn (as a bonus action) casts Misty Step as she is surrounded by silvery mist and then appears just past the barrier.  "Not where I hoped to be but not sure what else to do" she mumbles to herself as she then blasts the Elder Brain from where she appears.

Eldritch Blast.

Attack: 15 Damage: 13 + Bless 2

Attack: 22 Damage: 14 + Bless 1

Concentration saves: 8, 6

With a small puff of mist, Kaylyn appears beside Pax, leaving poor Derendil still hammering away at the invisible wall in frustration and confusion, wondering why he can't get in there as well. Kaylyn blasts the brain with her magic, and senses the powerful mind within pause to assess this new threat, and then lash out with a tentacle. Suddenly Derendil's fists no longer thud against any invisible wall, and Rhayna and Kirin can hear the sound of battle.

Tentacle Attack v Kaylyn: 20 Damage: 18 bludgeoning, and on a hit Kaylyn is grappled

Rhayna and Sekkar are next

Initiative order and conditions:

(A little extra while I have time...)

Up close the extent of necrosis on the brain's outer surface is more visible. The meninges look grey and dry, with patches of black oozing clear fluid, and the back half of one entire lobe seems to have collapsed. Yet as dead as the flesh appears, the creature's mental capacities are clearly powerful. No words are conveyed, but strong emotion is telepathically transmitted from the brain to all in the room; a sense of vast superiority, mingled with scorn at the audacity of these pitiful beings who dare challenge it; a feeling of invincibility, of having conquered death and never again being subject to the frailties of mortality; but all these thoughts slightly tainted with the knowledge that these are all gifts from another being, one even yet more powerful than itself, who even now maintains ultimate authority and control...

Sekkar knows better than anyone else that the powers of this brain are somewhat diminished. From what he knows of such creatures, their ability to detect and influence sentient creatures is immense, and yet the brain seemed to be unaware of their presence until the first time the mind flayers became aware of them, and scarcely seems capable of the kind of psychic control he would have normally been wary of. Clearly undeath has restored many but not all of the brain's functions.

For Sekkar the fight was simple: hit it until it's dead. Deader in that case but the tactic was the same - the ranger was on his mark and hit it with all he had.

BA: Hunter's Mark

Attack 1: Bless 4 + Attack: 16 Damage: 15 +1 (HM)

Attack 2: Bless 3 + Attack: 24 Damage: 17 +2 (HM) +3 (SA)

Rhayna will stay close to Kirin, and guard him like she did the menhir. (She will ready and action. If she sees an enemy on the east side, she will cast Guiding Bolt.)

Sekkar's flashing blade continues to carve chunks out of the brain's grey matter, which plop into the brine pool below it or splat onto the stone-like floor. Another tentacle lashes out at Sekkar in retaliation (Tentacle v Sekkar: 27 Damage: 45, and on a hit Sekkar is grappled) but for the first time, the emotion radiating out telepathically from the immense psyche is one of doubt and panic. There is a sudden stabbing in the mind as each in the room senses the brain shoot out a powerful telepathic 'shout', a call to all its mind flayer minions, as well as to the powerful being who restored it from death and under whose ultimate control the brain currently exists...

With only a few brief seconds of pause, something new begins to happen in the chamber. On the west side of the room, just behind the brain (to all except Pax who has flanked around to the rear) a shimmering in the air is seen. It intensifies and slowly takes shape, becoming a large black ellipsoid that reaches from floor to ceiling. A figure begins to materialise within the shape, darker still and growing more and more distinct and corporeal. The figure is vaguely humanoid, easily fifteen feet tall, with goat-like legs, a swollen torso, and a horned head like a skull. As the being begins to step out of the portal, two huge bat-like wings unfurl and fill the breadth of the chamber, and one arm raises a mighty wand - curved like a spinal column and topped with a skull.

A feeling of dread, as deep as the Abyss and equally as hopeless, washes over all in the room, not unlike that experienced previously upon beholding the presence of Demogorgon or Zuggtmoy. For in the midst of the battle a Demon Lord has arrived - Orcus, the Master of Undeath - and his contempt for all things living assaults the mind with a power so great that it makes the psychic attacks of the elder brain pale in comparison...

Everyone please make a DC 15 Charisma save

The presence of Orcus is felt, although he is yet to fully materialise in the chamber.

Orcus initiative: 9

Pax and Kirin are next

Kirin Cha Save: 7

"I do believe this is...well nothing something we should be closer to. Two demon lords.... When does that teleport work again? Doesn't have to get us out home, just out of here now,"  Kirin whispers as he looks for an exit. He holds up his staff aimed at the brain  adn cast Lightning Bolt from the staff.

Dex Save: 17

Damage: 30

Spell save: 6 + 1 Bless

Char save: 13 + 3 Bless

Dead or not, but that tentacle hit hard! The mark of the hunter disappeared from the creature, while the black decomposing tentacle wrapped around him,  leaving the lizardman breathless. Probably that was the reason why the appearance of a demon did not impress Sekkar much - he was too busy fighting for his life (and probably expected something like this anyway, and would bet - everyone did too).

CHA Save 18

Pax CHA save vs DC 15 : 18

Pax turns around, almost in slow motion to witness this horrible thing appear. Perhaps it’s luck, perhaps she’s “getting used to” demon lords, but she shakes off the fear. But this brain must die first. With any luck it may dispel the creature. 

Her tentacle rod lashes out as she steps away to try and avoid being flanked, stepping closer to the demon lord if necessary. 

Attack: 29 Damage: 18  

Attack: 28 Damage: 8

Attack: 19 Damage: 7

Rhayna CHA save: 17

Rhayna’s making sure she is covering the east side for enemies, then she can feel something is not right behind her, in the brain chamber, and she looks behind her…

(Kirin has a front roll seat on what's happening here)

While Kirin was able to resist the Demon Lord's psychic assault, the Cleric dedicated to Selune wasn't as fortunate. Rhyana was holding her spore empowered Zurkhwood Staff with both hands, and ready to face any thread coming from the eastern side. All of a sudden, she drops her guarding stance, and stands straight, in a relaxed manner. She cocks her head to the side then relaxes her shoulders. Her staff looses the empowering spores, and she holds it casually with one hand. Kirin sees that the weird human in front of him that was covered in this darkish gray moss & spores, with a simple gesture, she turns all those spores gradually into a very dark obsidian colored spores, that almost look very threatening to himself.

All sound of battle settles to the background now for Rhayna. She turns to the undead elder brain and smiles. "Fascinating..."