No Rest for the Dead in Avlor

CON save vs. Blight: 13

As Jarl's magic takes hold, the aboleth's slimy flesh begins to peel, flaking bits of its skin into the water (28 damage).

By the way, fantasy geometry note for folks on the rope, although you're 30-35 up, since we're using the simple version of distance where diagonals count the same as verticals and horizontals, your distance to the aboleth is just the larger of the vertical and ground distances (where you are now the vertical distance is the limiter, but if you climbed down 15 feet you'd be the same distance away as if you were on the ground, i.e., 25 feet).

Con save: 6

If Squiggles can use a reaction (not sure based on wave/prone) he will use Illusory Self (When a creature makes an attack roll against you, you can use your reaction to interpose the duplicate between the attacker and yourself. The attack automatically misses you, then the illusion dissipates.). 

Sure, you can do that. Being prone doesn't affect your ability to take reactions.

The three tentacles all strike their targets, however, one of the targets turns out to be an illusion. Balian has an uncanny ability to wriggle himself away so that the tentacle only lashes him in a glancing manner (7 damage). Shale, unfortunately, is not so lucky, taking the full force of the blow (16 damage). He also feels a bit nauseous (no mechanical effect yet).

Celric is up

During his turn, Flynt will use full movement to climb down and get to the floor. 

((Ooc: since I’m 5ft above Flynt I’m assuming I can’t move more than 5ft down. Can I cast from the rope? If I can I’ll move the 5 and cast sacred flame.))

Same as before you can hold until Flynt's turn if you want to climb down. You can cast from the rope (you can make it 15' if you also cast; 30' if you don't). I assume you're targeting the aboleth?

Celric shouts down to Flynt “Quickly brother!”

he waits for Flynt to move before taking his full movement down the rope but before that he lets loose a sacred flame at the Aboleth

15 dc 15 dex

Aboleth DEX save vs Sacred flame: 17

A ball of radiant energy descends from the top of the cavern toward the aboleth, but Celric's gestures high up on the rope were plainly visible, and the aboleth is prepared, ducking under the water in time to avoid the blast.

Brix, having had his rapier knocked from his hands, kicked across the cavern and then swept into the pool, draws a shortsword instead, and takes two swipes at Jarl.

Attack 1: 21 Damage: 10

Attack 2: 12 Damage: 6

Jarl, having been distracted by the sudden appearance of the aboleth, is hit by Brix's first blow (8 damage), but reacts quickly and manages to dodge the second.

Flynt focuses his energy on climbing, managing to get to the ground.

The aboleth quickly takes a swipe with its tail at Balian (using a legendary action at the end of Flynt's turn).

Attack: 21 Damage: 13

making a deep lash his arm (13 damage).

Squiggles takes a moment to right himself in the water after being knocked over by the tide, and attempts to back away from the aboleth; however, as he is backing up, the aboleth swipes at him with it's tail as well (Opportunity attack since it has a reach of 10').

Attack: 16 Damage: 15

and pressing deep into his soft gnome flesh (15 damage).

He will then cast Bigby's Hand at 6th level, clench his fist and unload an uppercut to the Aboleth (Forceful Hand), but despite the hand's incredible strength, it hits a squishy part of the aboleth's flesh, missing any important organs.

The aboleth takes another tail swipe at Shale (using its second legendary action at the end of Squiggles's turn).

Attack: 16 Damage: 11

but the cleric's armor protects him.

Edit: Rolled the wrong damage die for Brix's shortsword. Rerolling damage for the attack that hit: 8

Balian and Shale are up

Balian

Use potion as a bonus action, Dodge as my action, move to S10 and tread water.

Balian's survival instincts kick in as a tentacle grazes his arm, and he splashes away from the aboleth, struggling to turn in the water to face it.  He quickly downs a healing potion, and readies himself for whatever is next.

OOC - Balian never doled out those potions, work it into your post if you want one.

Use Greater Healing Potion: restore 14 hp

The water is too deep to stand in, but you can tread water and act normally, apart from it being difficult terrain. Note that the aboleth's reach is 10', so at S10 you haven't left its threat range yet; your disengage isn't actually doing anything, if you want to do something else with your bonus action.

      "I can look at Brix later. Just knock him out," Shale replies to Jarl. "Ow! Hey, we'll be with you soon enough you giant, slimy trout..! " The genasi retorts as the aboleth strikes out at him. He rights himself in the water and then swims to dry land. "Ooof, my stomach doesn't feel right. That last ration must have been bad." He speaks a quick word to provide himself with a bit of healing and hopefully relieve his indigestion.

(Movement: Up from prone and move to T9, Action: Dash [taking OpAttk if it has one] to W7, BA: Healing Word 8)

Balian rights himself and backs up, assuming a defensive stance.

Before Shale can act, the aboleth lashes at him once again with its tail (using its last legendary action of the round).

Attack: 29 Damage: 26

Shale is then able to regain his balance in the water, and swims away at top speed, pulling himself up to solid ground and out of the aboleth's reach for the moment at least (it used its reaction already this round, so no AoO). He channels the light of Selune into himself, healing some of his injuries. Unfortunately this doesn't seem to do anything to relieve his nausea.

The rock around the cavern suddenly begins to hum, and you feel something pierce into your minds... (Everyone make an INT save)

Int ST 18

(Holy Shit!! Honestly, who's wrong side did I get on. 3 d20s: 3, 6, 2, a 1 on the d4 for my heal, and then I get crit on... Come on law of averages. LOL) 

Yeah, definitely not your day dice-wise.

INT save (this should be fun): 12

OOC - I've adjusted my last post to quaff a potion.