Pax looks around at the flashing red. "We should stay together, or at least on the same level. I say we wait the 10 minutes for the servant. If that door is jammed, or locked and can't be opened, we'll be short two party members for the fight, and separated." 

"Let's wait then," did not argue Sekkar, signalled to Derendil and both took watch in the corner of the corridor leading to the ceiling hole (the one the group came from).

Kirin contemplates summoning his invisible servant... and then remembers how far it is back along the corridors and up to the next level, and how far out of range that will take him from the servant, which can only be a maximum of 60' away before being dispelled. Is there no other way...?  Quickly estimating the distances of the corridors and the height of the complex, and performing some complex three-dimensional trigonometry in his head, the wizard calculates that if he goes upstairs first and stands just outside the southern energy barrier when he casts the spell, he could summon the servant to appear right beside the machine, downstairs and out of sight but still within range of mental contact, and should be able to remotely instruct it in the necessary task sequence to open the barriers. (How sweet it is to be smarter than the average mind flayer!!)

((I liked the idea so much I had to make it work. The spell description doesn't say the servant has to be summoned within sight, just in an unoccupied space on the ground within range. So you can be upstairs and summon it to appear downstairs, as long as its within 60 feet. The distance is a bit of a stretch, but I think it's close enough to 60' to be legit. Since the spell lasts an hour without concentration, Kirin can use a bonus action on any of his turns to instruct the servant to raise or lower the barriers, or to open or close the trapdoor in the floor or the exterior door behind the brain.))

Assembling themselves upstairs, the group have just over ten minutes to prepare while Kirin casts the spell, instructs the servant, and waits for it to lower the barriers...

While waiting for Kirin to perform his ritual, Pax notes how odd it is that no other mind flayers have attacked. The flashing red is certainly some kind of alarm, but probably only a local alert, and not capable of signaling anyone far from the complex... and with no other corridors or rooms visible, it's possible they have already explored the entire facility, and slain every mind flayer that was left behind. However, three mind flayers is far too few for a colony with an elder brain, so there must be many more out and about somewhere.

While waiting, Sekkar has time to observe the undead elder brain through the barrier. It seems much more active than when they first saw it, and probably became more alert after the fight with the last two mind flayers. But not being truly alive... what might that do to such a creature? Would it lose its fear of death? Would that make it more arrogant? How much of its intelligence remains? Regardless, it must have been exceedingly powerful magic indeed to reanimate a dead elder brain. Who could have done such a thing? As far as Sekkar is aware, necromancy is not something mind flayers typically engage in.

Rhayna spends the available time making a few extra notes and cross-references in her journal. The experience in the mind flayer lair has been most educational, and quite unlike any other she has had so far. She wonders whether this famed library of Gravenhollow for which they were destined has any books on mind flayers, or whether any of her notes might be worthy of contributing to the library.

Kaylyn spends much of the available time concerned for Pensive. Even knowing that her familiar can be restored or re-summoned doesn't eliminate the distress she feels at knowing her trusted familiar is in such a poor state, and the feeling she has let down the faithful pseudodragon. Has she likewise disappointed her patron? Kaylyn's mind wanders as Kirin casts his spell. Huh. Kirin. Ki-rin. Was the elf named after the same kind of being as her patron?

((Anything you want to do prior to the battle, please declare it now, and roll initiative. Also decide who is waiting outside which barrier. Kirin must be outside the southern barrier to remain in range of the servant, but everyone else can choose.))

Kirin Iniaitive: 16

He's busy with his spell so no time for much else. I believe he should have his mage armor up still when before they teleported here.

Kaylyn chooses to stay near Kirin when the fighting begins.  She frets about Pensive and worries that she should have let Rhayna heal him, yet she knew the MFs and their creatures could do more harm to the others and she needed to keep that option of healing available for the others.  Poor Pensive...

Initiative 8

Too distracted to be ready to go.

Rhayna Initiative: 26

Rhayna will stay with Kirin and Kaylyn. She will imbue her Zurkhwood Staff with the power of her spores (Shillelagh), then she will empower her spores to make herself stronger (Symbiotic Entity). (She will be Kirin's bodyguard.)

(She will Bless everyone but herself on the next turn.)

(Symbiotic Entity - You can use Wild Shape to awaken the spores that infuse you to gain +8 temp HP, roll double damage dice for your Halo of Spores damage, and deal 1d6 additional necrotic damage when you hit with a melee weapon attack. These all last for 10 minutes, or until you lose all these temporary hit points, or until you use your Wild Shape again.)

Pax will stand in front of the other entrance to the room....ready to rush in. When ten minutes is about up, she casts a 3rd level Armor of Agathys.  The shadows around her roar back into being, swirling in darkness. She then readies a Eldritch Blast, the crackling power simmering in her hand as she focus' on blasting the thing as soon as the field drops. If it's tough to tell, she keeps a gauntlet pressed against it so she knows right away. 

Initiative : 17

Sekkar Initiative: 22, Derendil Initiative: 10

Neither Sekkar nor Derendil had anything to prepare. Both were on alert for potentially coming mind flayers and Sekkar did mention out loud that those blighted toad spawns preferred alive brains, so, there can be something else here. But since he personally counted mind flayers as the worst possible things, anything else would be an upgrade. So, after activating Dawnbringer (the sword could also keep some guard) he just waited to attack and destroy (as fast as possible, of course).

Elder Brain initiative: 19

At the southern door: (Rhayna, Kirin, Kaylyn)
With Kaylyn and Rhayna at his side, Kirin completes his ritual spell. He continues to stand motionless, eyes closed, sending mental commands to his servant but narrating proceedings to Rhayna (and Kaylyn who is only half-listening). Unable to see what the servant sees, but having meticulously memorised every detail of the instructions for opening the barriers and the exterior door, Kirin announces that the exterior door should be opening first. Sure enough, visible through the semi-transparent glowing barrier, the rear wall of the elder brain's chamber splits down the middle, folding back in on itself in a most unusually complex geometric manner. Beyond the new opening a wide dark rock tunnel can be seen, dead-straight and smooth-surfaced. Kirin announces that he is about to drop the two barriers, and Rhayna promptly activates her spores and casts her two spells.

At the northern door: (Pax, Sekkar, Derendil)
Dawnbringer softly hums a tune in Sekkar's mind, anticipating the battle. Her blade even emits a low audible buzz as Sekkar takes a few warm-up swings, cutting through the air and the building tension. Derendil flexes his broad shoulders, ready and willing to unleash his raw savagery on behalf of his 'friends'. Pax, with a grim set to her jaw and her armored fist resting on the barrier, watches the elder brain's tentacles writhe menacingly, wondering whether its first attack will be physical or psychic. Unable to follow Kirin's proceedings, nevertheless both Sekkar and Pax feel the moment when the blessing of Selune descends upon them from the out-of-sight cleric, a split-second before the barrier drops...

---------------------------------

Rhayna casts Bless on her first turn. (Does this target Derendil as well, or just the other four?) Since it's a bonus action cantrip, I'll also have Shillelagh start on the same turn.

Sekkar is next.

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

The brain takes up four squares on the grid. You can only target these squares with melee attacks. The tentacles have a 30' reach from there.

Rhayna casts Bless (lvl3), so it includes Derendil, but not herself. (I wasn't sure if I was allowed to cast Bless bc the other party is not next to her.)

(Symbiotic Entity takes an action, so Rhayna will use that feature on her next turn.)

(Folks, make sure to add the d4 to either attack or saving throws. I am going to assume there will be many saving throws to be had here.)

Quote from Wolfy911 >>

Rhayna casts Bless (lvl3), so it includes Derendil, but not herself. (I wasn't sure if I was allowed to cast Bless bc the other party is not next to her.)

(Symbiotic Entity takes an action, so Rhayna will use that feature on her next turn.)

(Folks, make sure to add the d4 to either attack or saving throws. I am going to assume there will be many saving throws to be had here.)

 Cool. Derendil is also blessed then. The targets only need to be in range, not visible, so you can Bless anyone in the party.

Symbiotic Entity lasts for ten minutes, which should be longer than this fight (or just about any fight), so I thought it better to have Rhayna activate it in the time before the fight, saving her from spending an action during the combat.

Quote from ZorgAlmighty >>

Symbiotic Entity lasts for ten minutes, which should be longer than this fight (or just about any fight), so I thought it better to have Rhayna activate it in the time before the fight, saving her from spending an action during the combat.

Cool. Maybe I will do that going forward for big battles. Thanks.

There were, probably, many ways to skin that particular cat but Sekkar knew only one - jump in and cut the foe into small pieces as fast as you can. So, he tried.

(To DM's decision - should be possible but up to you, of course)

Sekkar dropped the shield and shot the disgusting pudding in the middle of the room with his crossbow. Then picked the shield back and went in for the strike:

BA: Zephyr Strike

Action  Attack 1: (crossbow): Bless 3 + Attack: 15 Damage: 9 + 3 (Zephyr)

Movement: run to the brain

Attack 2: Bless 2 + Attack: 24 Damage: 14 +3 (SA)

Attack 3 (Ambusher): Bless 2 + Attack: 16 Damage: 16 + 6 (Ambusher)

Quote from BlueBayou >>

(To DM's decision - should be possible but up to you, of course)

Sekkar dropped the shield and shot the disgusting pudding in the middle of the room with his crossbow. Then picked the shield back and went in for the strike:

 

I'm afraid that's too much for one turn. According to the rules it takes an action to doff a shield, and another action to don one. Also it's a bonus action to activate the sun blade, so can't do that and cast Zephyr Strike. You can use two different weapons with Extra Attack though, including a pre-loaded crossbow. I'm not sure what the purpose of the crossbow is though if you are attacking with the sword afterwards. Let me know, but for now I'll apply the Zephyr Strike, and I'm sure all the attacks will hit so I'll apply the damage as is for now.

Sekkar charges in and starts laying into the undead giant brain with Dawnbringer, sending rapid shadows dancing along the walls from the columns and tentacles.

Nothing else seems to happen.

Pax is up next

Pax taps her boots together and charges into the room.  She rushes around the thing, attempting to get at it from a different angle from Sekkar.  

(She can move 70 feet, which seems like enough to get some flanking, especially with her reach of 15 feet, but...let me know.) 

The tentacle rod lashes out, three long ropey limbs flailing at her foe. (Not sure if she could have either a) assigned Hex to the creature or b) tapped her boots before the field went down ) but if either was ok, then she should be able to have hex on the creature. (vs Dex).  I'll add the damage, ignore it if she wasn't able to.) 

Attack: 19 Damage: 6 + 3 necrotic

Attack: 29 Damage: 10 + 3 necrotic

Attack: 21 Damage: 3 + 2 necrotic

Pax gets a mere fifteen feet into the room before slamming against something. Reaching out, she can feel an invisible wall separating her and the rest of her companions from Sekkar and the brain.

(Sorry to pull a fast one on you like that. Can post a different action if you wish)

The invisible wall runs South to North across the room, from floor to ceiling.

Kirin is next

Crashing into the wall unexpectedly, Pax crashes to a sudden halt. She growls and summons forth her magic and attempts to jump reality to the other side of the wall. (Attempt to cast misty step to behind the creature.)

(Danwbringer was activated before! You even played it out with humming. The reason for the shooting is to activate Zephyr and get enough speed to reach the brain, which is otherwise not possible even with Sekkar's 40ft movement? If everything is possible without Zephyr, Sekkar would rather use Hunting Mark)

Kirin seems to flex his fingers as he holds it up parallel to the ground as he chants pushes his hand out as what looks a ghostly copy of his hand shots out but it grow bigger and biggeruntil become a large transparent hand next to the elder brain. Kirin cast Bigby's Hand. Kirin makes a flickering motion with his hand which Bigsy's hand mimic at the elder brain. [Clenched Fist but saying flickering it in the head and its all head seem funnier]

Attack: 13 Damage: 19 Force

(Would Kirin know what the invisible wall is? I believe he can cast it himself if that helps)