Hildi wants to fire an arrow into the water at the chuul. But he knows about the problem with sight lines. Could he bring his mage hand about and use it to physically find the chuul and then aim near where he feels the hand is touching the chuul instead of where he sees the chuul? Since he can make the hand invisible, he has to be able to "feel" where the hand is instead of using his eyes, right? So, I see him getting out of the boat, using his bonus action to call his mage hand into being where he thinks he sees the chuul. Then, still using his bonus action, moves the hand around in circles to bump into the chuul. Then, he fires at the chuul, using the mage hand's location instead of what his eyes see.

I'm going to roll normally, and you can roll whatever else you feel should be rolled to make this right.

Movement: get out of the boat and move just enough to see the chuul underwater

Bonus: mage hand

Attack: 17 Damage: 6

Sneak attack?: 11

(Okay... Moonbeam... No problem targeting the Chuul. The tough part is not getting Jorin also.

I don't want to ask for a Perception check every round, but that seems to be the way to go for targeting.
The DMG lists that murky water has a 10 ft. "encounter distance", which is already the depth of the water.
If it is any distance laterally, it would be tough to distinguish Jorin from the creature.

Also, it might move under the raft (assuming it still has Jorin in its clutches). A 40' tall cylinder will get everything from the water floor up to the ceiling.)

(For the sight lines, I was thinking about placing an area-of-effect precisely enough that it gets the chuul but is still able to avoid the grappled Jorin. A bow shot doesn't have that problem.

For Hildigrim... Bows have disadvantage for underwater combat, so that applies for this arrow shot. Roll another to-hit, please.

He can Help, since Mage Hand won't be necessary.
Pascal will probably do a Frost cantrip, so the Help would negate the disadvantage for his ranged attack. Jorin is underwater, and won't be able to hear the Help. Drusk will be attacking chuul-1 at the surface, unless the party wants him to dive down and try to break the grapple.)

Hildigrim needs to make another to-hit roll, and decide if he wants to Help. And Quillary is up.

26

He'll continue to help Drusk.

(Moonbeam seems ill-advised, and Water Walk actually prevents Quillary from swimming down to help him, so…)

Quillary moves along the surface of the water to get a good vantage point, and gestures, and the chill of the grave attempts to grasp the chuul underwater with Jorin.

Action: cast Chill Touch at the chuul holding Jorin. 17 to hit, for 12 necrotic damage

(OOC: I thought this would be a tough encounter, with people being grappled underwater and getting paralyzed by the tentacles. A few bad rolls, and it becomes a cakewalk.)

Hildigrim's arrow hits for 13 damage. That is followed by Quillary's spectral hand grasping at chuul-2's head for 12 necrotic damage.
The water gets even murkier from ichor, and Jorin starts floating towards the surface, no longer gripped by the claw.

Drusk turns to chuul-1 to attack where Hildi pointed, and also moves the spiritual weapon.

• Attack: 26 Damage: 8  (only hits due to Hildigrim's help)
• bonus Attack: 23 Damage: 5  (miss)

Chuul-1 takes another 12 psychic damage, and tries to shake off the spell. WIS save: 12  (success, the spell is broken)

Jorin and Pascal are up. (Jorin will float to the surface, between his movement and the water-walk spell. He will be prone. He can use his Acton to attack with disadvantage, or he can stand up, but not both.) After they go, the next round starts with Hildi/Quillary/Drusk.

Jorin surges through the surface of the water like a breaching whale, roaring with delight after experience such a new form of combat!  He opts to continue this aquatic dance rather than try to get better footing on the uneven surface.

• Action:  Reckless Battleaxe!
  • First Attack: 19 Damage: 10
  • Second Attack: 15 Damage: 11
• Bonus Action: Equally reckless Moonblade!
  • Attack: 16 Damage: 13
• +6 Radiant damage to the first strike that connects
• +2 CRITICAL Radiant damage

The chuul comes to its senses, just in time for Jorin to breach the water surface like a terrifying great white shark. (Drusk: "We're going to need a bigger raft.")

Despite not being used to fighting in slimey water, the foothold from the water-walk spell (and his angry attitude) lets him strike 3 times for 54 damage.

Seeing how well combat is going, Pascal decides to continue using Fire Bolt. Attack: 17 Damage: 19

Between Jorin's assault and Pascal's firebolt, the last chuul explodes into pieces (as if a compressed air tank went off in its mouth).

Drusk gives Jorin a hand in getting to his feet.

Quillary is satisfied with the effect of her Chill Touch, but then wrinkles her nose at the smell of the slop clinging to her boots.

Hildigrim looks at the rusty miner's pick that is stuck in the west wall at the water line. He also looks around the cavern...
No magic within 30' in any direction (other than the party and their equipment).

Pascal is disappointed. "Those are truly foul creatures. Pincers of that size, and still no decent meat."

"That was an excellent battle!  Now, where is that giant fish monster, i have an axe to show it!" Jorin says, looking around the illusory fish creature.

Since Hildi was looking that way, I'm going to assume he watched the creature disappear in response to Quillary's spell.

Hildi tears his eyes away from the miner's pick to look back to where the three-eyed monstrosity had been before Quillary dispelled it. He can still feel the gaze of whatever has been watching them, but his brain is having trouble figuring out what, exactly, is going on. Was the other monster an illusion as well? Was that what his detect magic spell had detected during the night: more illusions? Whatever it is that has been watching them is clearly gifted, but how powerful are they ... really? He frowns. "It was nothing more than an illusion," he says, sounding almost disappointed. He looks at Jorin and the others, deciding they are relatively unharmed. Continuing, he explains, "Quillary's spell dispelled it. And I can't help but wonder if the creature we saw with the troglodytes earlier was just as illusory." He continues to frown, clearly frustrated. He feels the urge to yell out at whatever it is that is watching them, but he suppresses the impulse and turns back toward the miner's pick. Walking closer, he gives it a good luck.

Investigation: 10

Ouch!

Quote from DirtyDogP >>

Investigation: 10  Ouch!

(Yeah, but sometimes the DC is only 10.)
It really is just on old rusted pick. The only way it would have value is if it was enchanted, but Hildi's Detect Magic confirms that it is not.

Drusk does a Medicine check (with Guidance) on Jorin, since the thing tried to drown him in the slimey water. Medicine: 20
Drinking the corrupted water will eventually cause the drinker to be poisoned. Jorin did not (accidentally) drink any water when he was submerged, and contact with the skin for a modest amount of time will not cause any problems.

The Detect Magic will expire any moment. Still 3/4 of an hour left on the Water Walk.

(I put the map image in the spoiler, if you need a refresher. I have not updated the map.) You are in the cavern connected to the 2 water passages (middle of the map).

• You can continue examining the cavern.
• Could spend time doing more knowledge checks about the illusory monster or the Chuuls. Hildigrim is out, but Pascal could try Arcana, and Drusk could try Religion.
• Could continue to the passage to the NE of the cavern (where the illusory monster was).
• Check out other areas on the map that haven't been explored yet.

Quillary grimly shakes some of the chuul-chum off her boots. “We should continue down this branch and find what the illusion was protecting…” then she looks back at the others, “Or, we decided to leave it well alone, didn’t we.” She reluctantly turns back to look at the others.

"I agree, we came into this passage to explore and were rewarded with a excellent fight.  We should continue."

Id insinuation (archived) is concentration, so Hildi would have lost detect magic as soon as he cast that. I think the only thing that affects is the pick.

Hildigrim brings his mage hand into being and uses it to pull on the miner's pick to see if it will come free from the wall.

"Hildi, would you like a hand freeing that pick?" Jorin says, and will try to pull it free as well if the mage hand isn't able to.

Athletics 10

(OOC: Right about the Detect Magic. You'll need to spend 10 minutes if you want to Ritual cast it before going forward.)

Hildigrim obtains the pick. With his blacksmith background, Jorin decides to look at it as well.
You look at the rusty pick up close and have time to examine it.
You confirm that it is an old, rusty miner's pick. No magic, no writing or hidden compartments, nothing special.

Hildi feels detect magic has been some use, so he would appreciate it if they could take 10 minutes for him to ritual cast it.

(I assume that Quillary will also take the opportunity to recast Water Walk.)

Jorin and Pascal can keep watch on the surroundings, as Quillary and Hildigrim cast their ritual spells.

Drusk will meditate (and use Guidance) to try to remember if the temple priests talked about any of these weird creatures during acolyte training. Religion check: 11

I'm sorry yes, Quillary would do that, to re-up the hour's duration.

While casting his spell, Hildigrim replays in his mind the most recent events. His natural curiosity — the part of him that normally drives his every thought — continues to war with this new fear that his friends might be killed. However, while the fear has been easily overtaking his curiosity since the threat, curiosity is now beginning to get a word in edgewise. Are all of the three-eyed monsters illusions? Is the voice an illusion? No, he knows they've been watched for quite a few days, and the voice knew it as well. The voice must have some power.

Hildigrim stands once his detect magic spell has been cast once again. Volo hoots from his shoulder. The halfling turns and looks back toward where they had seen the trogs and the three-eyed monster, his curiosity pushing him to suggest going back. But the fear fights back, and he looks away, ready to proceed forward.