No Rest for the Dead in Avlor

     "Didn't that dead fish man say that it controls people in the town? Maybe if we find a way to break that charm we could draw it out?"

Quote from Esker >>

"Many creatures you would call evil dwell in the underdark. As far as I am aware, the one I served controls those nearest your surface city."

 Just so you don't wind up heading in a direction because of an ambiguous antecedent, the kuo-toa meant "those [evil who dwell in the part of the underdark] near the surface city"; not necessarily people in the city itself.

“I could summon a water elemental to drive it out,” Jarl suggests, with some degree of hesitance. “But it would only be for an hour, and I can’t give it too complex instructions. I’d feel more confident about it being a useful option if we had a better idea of how far away the aboleth is, and the geography of the passageways. Some of that I could probably learn by communing with the earth, which will take a little time. It’s also possible that just the passages are flooded, and past that it’s dry again.” He thinks. “Though not too likely. What water controlling spells do we have? I’m afraid I wasn’t planning on going swimming when I prepared my spells this morning.”

"I could burn it all if that helps"

     "Going to be hard to burn a stoney, wet cavern, no?" Shale moves away from the dead kuo-toa. "Though I may have mistaken its meaning." He paces the cavern for a moment before eventually calling out, "Are you still wanting to discuss our terms of service?!"

"Sure, but it would be fun"

"...ervice?" Shale's voice echoes in the cavern. Then silence.

After a minute or so, a tentacled figure appears in the entryway. Everyone except Squiggles hears in their minds, "I see you've defeated my servants. You seem to be in a stronger position than you were a few minutes ago. Why would you now be entertaining my offer?"

     Shale turns and jumps a bit at the sudden reappearance of the creature. He begins to whisper a quick prayer for some assistance to determine if the creature is actually in their presence (Perception 15). "Your 'servants' here were unable to persuade us otherwise, and just wanted to let you know that you're next."

Sussing out illusions is normally investigation, rather than perception. You can't tell by looking at it whether this is the aboleth in the flesh, or a projection.

You hear a horrible raspy noise in your minds that you take to be what passes for laughter to an aboleth. "Am I? Well, I appreciate the advance warning. Tell me, have you come all the way to the underdark just to find me? What concern am I to you?"

"I see I've flummoxed you into speechlessness. Very well. I will be in my lair preparing for what I'm sure will be an overpowering invasion." The aboleth turns to head back out of the chamber.

As the creature turns to leave, Shale pulls a spear from his back and launches it. "We're not done here!"

Attack 9, Damage 8

Initiative if needed 11

Squiggles, uncertain as to what just went down, reacts to Shales shout by casting Wall of Stone to prevent the Aboleth from fleeing

Flynt knocks an arrow and fires it at the aboleth. 

Attack: 13 Damage: 10

Celric casts Hold Monster on the Aboleth, "This is where you will stay."
DC 15

I'll make initiative rolls for you to see what order the above responses happen in.

Shale: 20

Squiggles: 5

Flynt: 13

Celric: 17

(Balian: 24)

(Jarl: 7)

Shale's spear is right on its mark, sailing toward the aboleth's back... and through it, causing it to disappear just as before.

Since Shale's attack happened first, Squiggles and Celric don't need to use those spell slots unless you would still decide cast them after it disappeared.

Squiggles stops mind incantation as the spear goes through

Celric stops cold seeing it disappear. He does however pull his shield back on to be ready just in case. 

Jarl sighs. “It taunted us like this once already, of course it was doing it again. We’re going to have to do a bit more than that to actually draw it out.” He looks in the direction the aboleth ‘disappeared.’ “Let’s try going in a direction other than that one, too, if possible. Do we have a way to make our own passage? Or to remove the water?”

“I do not have anything like that prepared. But perhaps on the next nights rest myself and any other magic users can work together on making ourselves better utilized for our current situation.”