Sehaethas is about to draw his crossbow and approach, but now that things are finally looking interesting Kylaski takes over. Not taking the time to change from Sehaethas' form, she dashes out to the end of the real dock and tries to send an Eldritch Bolt into the elf killer.

25 to hit (nat 20 - crit), 11 force damage

Damros stays in the shadows and looks up and down the dock area hoping no stray person shows up to raise an alarm.

{game log}    Perception Check:   Nat20 + 4 = 24

As the drow has not managed to draw his shield while falling into the dock, Skye just hits. Xaul gets a solid hit, unlike his patron's tentacle. Kylaski gets a powerful blast on him.

Damros, no one is around to have witnessed this.

The figure wants to swim to the surface, and therefore takes two swipes of their scimitar at Skye. As underwater and not a piercing damage weapon, they have disadvantage.

First hit is 14 for 8 damage. Second hit is 10 (a miss). They do not attempt to push past to the surface. Currently they are about 10ft underwater, and Skye is therefore 5ft underwater, next to Xaul. 

Everyone is up again

Skye spins as the scimitar strikes her, doing what she can to absorb the blow on her leathers. For a moment she surfaces, her feet kicking out at the Drow, and takes the opportunity to cast her Witch Bolt, before diving back down with a fresh unending breath to keep the killer underwater. Even bogged down as he is, his shield hints at his strength and agility and she forgoes the opportunity to attempt a grapple as more risk than its worth. 

[Attack - 11 (miss)]

Xaul swims further away from the murderer, until he's a full 30 feet away, before repeating the same strategy of graceful emerging from the depths to cry his verbal component aloud, and then drops back down into the cover of the water as the sea is illuminated by his spell surging across the water's surface towards them. He then also commands his tentacle from a distance, but it is still struggling to maintain proper form within the mass of water, getting mixed in and swashed about by the existing waves. [Action: 12 + 4 = 16 to hit with Eldritch Blast, 2 force damage on a hit, Bonus Action: 2 + 4 = 6 to hit with Tentacles of the Deep, 2 cold damage on a hit]

Certain the dock area was empty of random visitors, Damros works his way up next to  Seha/Kyla at the end of the pier.   Looking down into the dark waters, he spies the flashes of arcane and eldritch energies being loosed upon the elf killer.  

As the killer starts to rise toward the surface, Damros will let loose his own Eldritch Blast at the underwater figure.

{game log} To Hit:  26         Dmg:  13 pts force     (Eldritch Invocations - Agonizing Blast and Lance of Lethargy)

"Let's see if that slows him down..."

Xaul hits again, his tentacle still missing. Damros slows down the figure a bit more. Sehaethas is sill up

Damros turns to Sehaethas, "How long do you think they can stay underwater?"

Sehaethas answers, "Maybe a minute or so, being shot at has to make it harder to hold their breath. The hard part will be fishing them out if we want them alive."

Meanwhile, Kylaski uses his body to cast another Eldritch Blast at the elf killer. pushing them back down again.

Agonizing and Repelling Blast to cause extra pain and push further away from air.

12 to hit for 9 force damage.

So I wrote a reply to this earlier. But I think I closed the tab without sending. 

To summarise, Kylaski misses. And so does the drow, on his first hit. The second hit (against Skye) is a 22 to hit (with disadvantage) for 9 piecing damage. He's not quite desperate enough to try swim past for air, and is holding up quite well. 

(Skye is looking very roughed up HP: 2/19). 

Also, forgot to say, you are all up

Xaul is irritated to see from afar that the drow is doing well, and holding his own against the party, but he resolves to continue on, thinking I can stay hear a lot longer than he can...there's one of him and four of us...or is it five? Anyway... He blasts another soaring green beam across the water's surface once more, which strikes the foe cleanly [Eldritch Blast : 15 + 4 = 19 to hit, 9 force damage on a hit], before commanding his wave to strike down on the attacker again. [Tentacle of the Deep: 13 + 4 = 17 to hit, 8 cold damage on a hit and the drow's speed is reduced by 10 feet until the start of Xaul's next turn]

Damros peers into the dark waters.  "Damn, I wish I had some magical light spell.  I can't see much in these dark waters."

He continues watching - waiting for a separation of bodies so he can get a clean shot without hitting his companions.

(Eldritch Blast (with all it's extras) held until a shot presents itself.)

The drow is now looking injured (<50% health). Skye and Kylaski to go

Kylaski tries twitching her tail, but since she's still in the form of Sehaethas, nothing happens. Using her own voice, Kylaski sends an Eldrich Blast at the elf killer to again push them deeper into the water.

22 to hit for 10 Force damge and push 10 feet deeper.

Skye lets the cord of magic binding herself to the Drow spark with a swirling current of magic that snakes through the water unnaturally, bubbles rising from it path.  She stabs with her spear, but hits the shield. 

(Witch Bolt BA: 8 Lightning damage. Action: Spear, attack roll 4)

Damros, seeing a bit of illumination (thanks to Skye's efforts) releases his Agonizing Blast Eldritch Blast

{game log}  To Hit:  19      9 pts Force dmg

Ooo, I do like the new witch bolt that lets you auto do damage even when you miss. (and don't forget it is [spells] with an s for the 2024 versions)

The drow is panicking now (spoilers, it is on 4hp) and is determined to take at least one of you down with him. He attacks skye twice with disadvantage. First is a 14 to hit for 8 damage (I think Skye is down). He then tries hitting her while she is down (straight roll, as unconscious but underwater) and gets a nat 1.

Everyone is up (AC16, 4hp. If you hit and kill, feel free to narrate the killing) 

(Ahh thank you. I had missed that about spells. And yeah, the slightly more limited, more boss focus but more easily broken style of a ‘hex’ that it has now is cool) 

(Skye is down (and underwater).)