For Grixx's action, now that Charles is with him and Drago is in sight.  Grixx will turn to look to Jury to ensure they cross the water safely.

((I assume Charles' attacks would be at disadvantage from the Fog Cloud and the lack of light?))

If Charles can see the creature, he will make his two attacks, with the +1 magical bullets loaded:

Attack: 15 Damage: 11

Attack: 18 Damage: 7

If Charles can't see the creature, he will ready on the trigger of being able to see the creature clearly.

Drago flies towards the sand, drops Charles off and continues to hover above water nearby. Suspicious of the opaque fog in the center of the room, Charles readies his pistol to fire if a creatures comes out of it (just to be clear, the fog is above water). 

LEGENDARY ACTION (end of Drago's turn)

Without anyone noticing except Jury, the underwater creature attacks Law once again. This time, Jury knows an attack is imminent, and Law benefits from his mental link to his master to no longer be completely oblivious to the threat. (i.e. no advantage).

-> Tail swipe (melee weapon attack) vs. Law: Attack: 22 Damage: 17 bludgeoning. 

-> Undead Fortitude - CON saving throw {DC 22}: 12. The tail attack obliterates Law. The impact is so brutal, the zombie's body breaks into several distinct parts fouling the water even more.

(I'll resolve Grixx's turn next).

Grixx prepares to aid Jury in his crossing, in case the wizard looks like he'd need help

(OOC: We'll call this a readied action to help. Jury can use this help as he wishes in his next turn, as long as it entails crossing the water as per Grx's specification. I'll be expecting some flavor text describing such help. I'll allow Grixx to move out of turn if the flavor text is good!)

LEGENDARY ACTION (end of Grixx's turn)

The water creature will search for its next victim.

-> Detect - perception check: 19 vs. Order's stealth check: 15. The creature is in its environment. It has no trouble finding the second undead.

(Lair action next)

LAIR ACTION

At the edge of the fog nearest to the party, an image begins to form (Drago, Charles, and Grixx need to make an INT saving throw {DC 14}. Jury and Verdict are not affected).

Drago: 14 -> SAVE;
Charles: 16 -> SAVE;
Grixx: 6 -> FAIL.

Grixx begins to see the shape of a 10-foot men, muscular to the point his skin seems to be ripping apart, forming at the edge of the fog, just 15 feet away from him. He caries a bronze shield, a longsword, and a closed helmet that imitates a horse's mane. The figure is otherwise unarmored, and covered in the dried blood of its enemies. It speaks to Grixx: "I COME TO PUNISH THEE FOR NOT FEEDING MY BLADE". The figure approaches the barbarian and swipes at him with its longsword (Grixx takes 3 psychic damage).

(OOC: Grixx is under the effect of Phantasmal Force. He is the only one that sees this image. He has no idea it is only an illusion in his head. I recommend reading the spell carefully and behaving accordingly).

(OOC: Charles readied action does not trigger because he does not see the image. Also, it'd be hard to snap Grixx from this sudden holy appearance so he could actually preform that readied help action to benefit Jury...)

(Jury and minions are up).

(@All: Guys, when readying an action, please go ahead and roll the necessary die assuming the action will be triggered. It if is, I'll already have the rolls to consider. If it doesn't, I'll simply ignore them).

Verdict flies to help Grixx escape his mental trap, although Grixx would try to rationalize the illusion (as per the spell description).

Order moves and prepares to grapple.

Jury moves and blesses.

LEGENDARY ACTION (end of Jury's turn)

The underwater creature swipes its tail at Order (grapple does not trigger yet because the tail attack has reach).

-> Tail swipe (melee weapon attack) vs. Order: Attack: 18 Damage: 13 bludgeoning. A considerable hit to the head.

(Nothing happens in the next turn. Underwater creature is next. Do you guys know what it is yet?)

The creature will simply perform another swipe at its current target from 10 feet away and deep into the pool of water, noticing how easily hit the zombies have been so far.

> Tail swipe (melee weapon attack) vs. Order: Attack: 13 Damage: Unable to parse dice roll. bludgeoning.

-> Undead Fortitude - CON saving throw {DC 18}: 6. Order manages to resist the shear strength of the blow, and is reduced to an inch of its un-life.

END OF INITIATIVE ROUND 1

INITIATIVE ROUND 2

26. Charles - attack ready, but about to expire at the beginning of his turn, also blessed;
22. Drago 
20. Grixx - affected by Phantasmal Force, also blessed and helped by Verdict;
19. Lair - Phantasmal Force damage occurs in this turn;
16. Jury - concentrating on Bless, also blessed. Order has a grapple ready to go.
15. W 
12. A - concentrating on Phantasmal Force

UPDATED MAP (zoomed in a bit; only Grixx sees the large deity image)

 (Charles, Drago, and Grixx are up).

Grixx with his eyes as wide as saucers do not see or hear anything else but Tempus himself.  The legends had told of this exact appearance and from the sword and now the appearance of the Deity Grixx is certain it is indeed Tempus.  Grixx is horrified as his whole strategy in battle is to fight with the thought that Tempus would protect him.  Now, that protector stands before him threatening him.

"Te... Te... Tempus!  My Mighty God! Oh Tempus!"

Grixx drops his sword falls to his knees and frantically picks it back up again.

"Please, have mercy!  Please!  I am so thankful for this swo.. gift!  Please my god!"

Grixx now resembles a toddler fearing being discipline rather than a battle-tested Barbarian.  Tears begin to flow and Grixx forces out the hysterical words.

“Let me prove myself!  I was just searching for battle just now to feed your blade! One more chance is all I need!  I will not fail you ever again!"

Grixx holds his blade in front of him flinching behind as if trying to hide as much of his ashamed body behind it to stay out of reach of his mighty protector.  

Grixx cowers and takes the Dodge action.

(@Grx: Beautifully done. Inspiration token to you, sir/madam!)

(@Lerus and Grx: I just figured out what I'm going to do next turn. Your sacrifice will not be in vain Grixx.)

((Not much a floating gunslinger can do.))

He will use the ready action again. On the trigger that he will shoot once he is able to see the creature.

I'll put the attacks in this spoiler:

 ((Jesus Christ, a 31? Oh my.))

Drago quickly realizes that this dark room full of water is not going to be easy to manage.  He throws some dispel magic at the fog cloud hoping to make it go away.  He is hoping Grixx will come to his sense on his own soon.  He moves next to Jury (30 ft out of my 50 ft of movement) and says "Grab my leg."  He then tries to help him get to the sand bar (whatever movement I can add to Jury, with the added weight it wouldn't be the full movement but with Jury trying as well you never know :-) )

Charles refreshes his readied attack (OOC: Them rolls!)

Drago dispels the fog and a readied action is triggered. A slender elf of pale skin awaits in the middle, and without missing a step it lets loose a Cone of Cold ({DC 16} CON saving throw for Drago, Grixx, Charles, and Verdict. Adding them at the end of the post so I don't get the "manipulating" message)

Drago then flies over to Jury and offers help. (OOC: Jury can move 10 extra feet next round if he grabs the elf's leg. This is on top of however much movement Jury can get out of swimming/difficult terrain in his turn. Drago would move along with Jury, so effectively Ori can choose where they'd both land in his turn).

Charles' attacks trigger against the elf. (Pale Elf - CMD: 22). 

LEGENDARY ACTION (end of Drago's turn)

Surprised by it's target's resilience, the Aboleth will swipe its tail at the zombie one more time:

-> Tail (melee weapon attack) vs Order: Attack: 24 Damage: 15 bludgeoning.

-> Undead Fortitude - CON saving throw {DC 22}: 18. Order is the MVP - back to 1hp.

CON saves:

Drago: 17;
Grixx: 16;
Charles: 22;
Verdict: 7;

Those who fail take 30 cold damage. On a success, you'd take half as much. (That has to be the worst Cone of Cold roll in D&D history!) 

Almost forgot to add the bless dice for Charles and Grixx. Charles can add 1 to his save above and Grixx can add 2.

Being shot by Charles, the pale elf will let out a rhyme in a strange language freezing the air around the gunslinger (reaction, DC 16 CON saving throw: 30 - Damn! Charles warm breath is enough to keep him warm).

(Also, now that the Pale Elf is in the fray! at the beginning of Grixx’s turn, he must make a {DC 13} WIS saving throw: 11 - the barbarian is considered charmed by the Pale Elf. While under this effect, Grixx has disadvantage on ability checks and saving throws, but can repeat the saving throw at the end of each of his turn, ending the effect on a success).

In his head, Grixx interacts with Tempus, but the others only see the barbarian cover behind his sword. 

-> end of turn WIS save: 10 (see the last roll of this post).

LEGENDARY ACTION (end of Grixx’s turn)

Surprised osce again by it's target's resilience, the Aboleth will swipe its tail at the zombie one more time:

-> Tail (melee weapon attack) vs Order: Attack: 28 Damage: 14 bludgeoning.

-> Undead Fortitude - CON saving throw {DC 19}: 18 - He fought the good fight, but now he isn’t chucks.

-> I’ll add another roll here’s for the end of turn WIS save to account for Verdict’s help: 14 plus bless of 3 - the barbarian is no longer affected by the Pale Elf’s charm.

-> and here is the bless die for that Nat20 that Charles rolled at the beginning of this post: 4. - LOL, of course it would be a 4.

LAIR ACTION

Tempus appears to swipe at Grixx once again, this time silently (since there is no attack roll, the dodge unfortunately doesn’t help. Grixx takes 2 psychic damage but rationalizes it as slashing).

(Jury and Verdict are up).

(OOC: The pool is 60 feet deep, and it's considered difficult terrain because he does not have a swimming speed. Hence, he'd need 120 feet worth of normal movement to reach the bottom where Order was, which is exactly what you get with haste and dash).

After shouting some orders, Jury gains new impetus and dashes underwater. Swimming as fast as he can, he eventually finds himself face to face with the Aboleth. The creature's large mass only visible due to how close the warforged has gotten.

LEGENDARY ACTION (end of Jury's turn)

"At... LAST" Jury hears in his head.

-> Tail Attack (melee weapon attack) vs Jury: Attack: 14 Damage: 15 bludgeoning. The creature swings early, missing Jury.

-> Retroactive {DC 13} WIS saving throw at the beginning of Jurys turn: 24. Nothing happens.

The Pale Elf gestures and chants, casting Ice storm (Charles, Grixx, and Drago must make a {DC 16} DEX saving throw. I'll say Grixx cannot see this coming, as he is completely taken by the image of Tempus).

Charles: 10;
Grixx: 22;
Drago: 13.

All fails - You each take 7 bludgeoning, and 15 cold damage.

... but, Grixx's dodge action would still be in effect so he gets advantage after all. Here's another roll for him: 20 - still a fail, unfortunately.

The Pale Elf will then begin retreating.