Gerrard:

Gerrard fights back against the mental command from the horror and the suggestion from Kei, trying to focus on what he has seen up until now in their investigations, fixing his mind on the vision of the mind-controlled murderer's visions of the kracken destroying the city.

Constitution Save:  25

(If that is a fail, Gerrard tries to fight through the effects with Indomitable:  8 )

(The DC was 14, but Gerrard's indomitable nature manages to stave off whatever this mucous was trying to do to his water-filled lungs.)

The filthy water around Gerrard seems to boil with pure angry energy. Gerrard: Make a DC 14 Wisdom saving throw or take 5 psychic damage. 

The creature suddenly withdraws, vanishing into a submerged tunnel at the far end of the pool. (Disengage, swim west 40')

Kei notices another pair of legs step into the pool behind him. At least one of the cultists has evidently returned.

Everyone may act!

Kellen

(Initiative: 15)

(Portent rolls: 105)

An tall, angry looking half elf sporting a...mandolin? comes into the water, looking around for the creature.

Perception: 11

Gerrard:

Not wanting to breathe in the water yet, Gerrard doesn’t directly answer Kei.  Instead, he swims rapidly after the departing aquatic horror.  

Realizing he can’t reach the creature with enough time left to attack it, The young noble engages it, dashing just past it and trying to block its way and prevent a further escape.

(Move and Dash to engage and try to get on the far side of it so it can’t just disengage and swim away again down the tunnel.  Still planning to riposte if it attacks and misses him)

(Gerrard’s Will Save:  9 )

A half-elven cultist has entered the water near Kei.  Kellen sees nothing but the two strangers underwater.

Initiative: 
√ Kellen 13
Kei 9
Gerrard 9
Thing 8

Gerrard saw the creature retreat down a submerged tunnel. (It is about 40' from Gerrard right now, although Gerrard has lost sight of it in the foul gloom.)

Kei and Gerrard may act next!

Gerrard swims forward into the gloom, catching up with the retreating horror, but unable to get in front of it to block it. (A large creature, it completely fills the ten-foot-wide hallway.) (Gerrard: make another Constitution saving throw as you again encounter the awful mucous surrounding the creature. It seems to penetrate his pores; holding his breath barely slows down whatever is trying to affect him.)

Kei is up next! He has lost sight of both Gerrard and the entity.

Gerrard Constitution Save:  24

Kei 

Kei sees the faint glow that marked Gerrards position slink off into the distance. As his friend moves off another being enters the pool but Kei has no time to waste in figuring out if they're friend or foe. He pays the newcomer no heed as he races after his friend. 

((Kei too will swim and then dash to catch Gerrard. 80' of movement))

Kei swims deep into the pool and down the gaping tunnel until he can see Gerrard's glowing knife once again. He approaches within five feet of Gerrard, ten from the creature.

Gerrard senses Kei tensing with pain again. (Kei - take 12 psychic damage.)

"A temporary truce perhaps,"  buzzes the voice in Gerrard's head. "Allow me to explain. Together we will save this city."

Tentacles writhe out, seeking to grapple Gerrard.
Tentacle Grapple Attack vs Gerrard #1: 24. On a hit: Gerrard must make a DC17 Strength or Dexterity saving throw or become grappled, and must also roll a Constitution save vs. mucous.
Tentacle Grapple Attack vs Gerrard #2: 14. Hit: Gerrard must make a DC17 Strength or Dexterity saving throw or become grappled, and must also roll a Constitution save.
Tentacle Grapple Attack vs Gerrard #3: 29. Hit: Gerrard must make a DC17 Strength or Dexterity saving throw or become grappled, and must also roll a Constitution save.

The waters surge violently. Everyone in the water: Make an a DC 14 Strength saving throw or be knocked "prone" in the water. 

Everyone may act!
(Kellen is about 45' from Kei, an 55' from the creature.)

Gerrard:

Deflecting the first attack with a double swipe of his weapons (Defensive Duelist), Gerrard cannot avoid the second and third tentacles.  He tries to squirm away before they can find purchase on his flesh!

Dexterity Save 1:  7

Dexterity Save 2:   10

Wrapped up in the tentacles, Gerrard becomes grappled!

Constitution Save:  18

Constitution Save:  9

Kei 

Strength save 15

Gerrard becomes tangled in tentacles, but he finds his lungs relax, welcoming the filthy water. His skin is becoming somewhat translucent and slimy. (He can’t regain hit points unless he is underwater, and when he is outside a body of water, he will take 1d12 acid damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.)

Kei is knocked upside-down in the black water.

Kellen should also roll a DC 14 Strength saving throw or be knocked "prone" in the water.

Everyone may act!

Gerrard:

”I will not be corrupted by the likes of you!” screams Gerrard defiantly in his thoughts, visualizing the rotting corpse of Mister Dory and his ignoble death.  The young noble slashes and stabs with his weapons, desperate to kill this dangerous creature and save his friends!

Bonus action Feint for Advantage on first attack

Scalding Dagger first Attack:  Attack: 21 Damage: 20

Scalding Dagger Attack:  Attack: 25 Damage: 12

Scalding Dagger Attack:  Attack: 27 Damage: 14

Kellen

Strength save: 13

Kellen

Kellen starts wading into the pool, but can't keep his balance when the water starts surging. What the hell am I doing he thinks as he starts looking around under the water for the fools who jumped in after the Whisperer. I can't see a thing in here...

Kellen creates 4 orbs of white light leading down towards the tunnel. He'll start swimming that direction as far as he can.

(Orbs are 20ft away from each other until the get down to the tunnel)

Kei 

Head over heels over head, Kei is tumbled as if inside a butter churner. He is momentarily disoriented but soon hones in on Gerrards glowing sword. Pain ripples throught his body but he clears his mind long enough to recall the movements for a spell he hopes will prevent further injury. Swirling shadows forms mesmerising shapes surround Gerrard and his prey. They disappear as quickly as they formed.

Gerrard make a WIS 17 save or be incapacitated

Gerrard:

Through the mass of tentacles grappling him in the roiling scrum, Gerrard briefly sees the glow of his rapier reflected in Kei’s eyes.  Hoping his friend has come to help in the battle, the young noble grins when a spell effect of some kind settles over him (and possibly the creature grappling him?)

Wisdom save:  8