Another Duergar skeleton collapses into a heap of bones after a well placed shot from Minty. 

Initiative order: 20:S2, 15:S4, 14:Rizo, 10:Minty, 8:Spark, 8:Chuul

Map:

Spark runs forward to Q19 and launches two Magic Stones at S2 (or the second at S4, if the first kills S2).

Bludgeoning vulnerability included in these damage rolls:

Attack: 17 Damage: Unable to parse dice roll.

Attack: 28 Damage: Unable to parse dice roll.

The chuul Dashes around the chasm to R24, ready to crush any skeletons that survived.

Trying the damage rolls again:

22

19

Both skeletons fall victim to Spark’s loaded magic stones, leaving the cavern in a tense silence for a few seconds. Then you notice a rhythmic, dark chant in the distance, which suddenly stops at the chuul’s advance.

A cackling laughter echoes through the walls and a spectral hand floats over the ridge and had aimed for the chuul’s neck but missed by a narrow margin due to the chuul being to a large part still behind the highest part of the crest. “Hahaha! You’re hiding behind this walking fossil, eh? That won’t save you! I know you. I’ve watched you! You’re no match for my power! And were not even before I found this delightful source of power!”

on perception check DC 12 read this:

The voice adds more discrete, “Just a little bit longer, just a few minutes. Don’t let them disturb me! I command you!”

 

Perception: 18

(Are we still in initiative?)

(For now, yes. If you withdraw, you might get out of initiative, that’s an option.)

(Does that mean it's Minty's turn?)

(For simplicity you can act together. Everyone else, that is to avoid saying who or how many opponents there are, has their initiative outside your 12-8 window.)

Spark Dashes around the chasm to V26 and uses her bonus action to make more Magic Stones.

The chuul moves forward to R26, ready to either attack or retreat depending on what it can see from there.

Perception: 27

Seeing that Spark intends to push on, Minty moves forward as well (To U30 using move and cunning action dash).  Not knowing what awaits, he prepares to take evasive action (taking dodge action).

Minty and Spark hurry over the crest towards the voice, and the expected position of the gateway. Two figures are standing next to an elevated platform on which rests a tortle shell that emits a faint blue light (to Minty it appears that the shell is engulfed in blue, ghostly flames). One of the figures is the Tiefling that you've found outside the cave, the carpenter's apprentice, and he appears to be performing some sort of ritual on the relic. The other figure is a Flesh Golem that, upon seeing your approach, lumbers towards the chuul (or Minty if the chuul does retreat) and attacks with its fists. Curiously, there is an iridescent light tether originating from the golem that winds through the air towards the Tiefling, where it widens into a surface, and into an almost transparent bubble encompassing the Tiefling and the entire relic.

 (I'm assuming the chuul dodges, too. You can choose whether it retreated or not, and if the attacks aim at Minty or the chuul.)

Attack: 13 Damage: 13

Attack: 11 Damage: 17

Map:

The chuul remains on the ridge and dodges.

(Whose turn is it now?)

(Sorry, forgot to mention this. It’s your turn, that is Minty’s and Spark’s. When you both did what you want - in which order you prefer - then, the other side will act.)

Let's see how much Spark knows about flesh golems.  Arcana: 5

Not knowing anything about the abilities of the flesh golem, Spark advances to T27 and launches two Magic Stones at it from her sling.

Attack: 17 Damage: Unable to parse dice roll.

Attack: 29 Damage: Unable to parse dice roll.

The chuul attacks the flesh golem with its pincers.  These attacks do no damage since they are nonmagical, but grapple it on a hit.

Attack: 16

Attack: 10

If the chuul successfully grapples the flesh golem, it will carry the golem (at half speed since it's only one size larger) to Q22 and try to drop it into the chasm.  (I don't know if this counts as a free object interaction or if it will require another action next turn.)

Trying the damage roll again (for the first attack, since the second one missed): 3

That didn't work as expected.  Add 4 damage, and subtract 2 damage from the lightning absorption.