Feldspar thanks Drusk and breathes a sigh of relief before walking inside, planning to walk up on the ceiling or wall to at least confuse whoever's inside and avoid anything that might be on the ground. She won't be the first to go in. And she'll cast mirror image as soon as she sees someone/something who's not a captive guard. The duplicates walk all around her, some on the floor, walking normally.

She doesn't take point, but won't be the last one in. 

Jorin says, "Quickly now, before they realize their sorcery has been foiled." and takes off at a fast pace towards the cave entrance.

This room is dimly lit and clearly has been occupied for some time. In the middle of the room, addressing a circle of runes carved into the floor, is a creature of nightmare – half drow elf and half giant spider, reading from a thick tome decorated in webbing and eyes. Eight humanoid figures are bound in ropes (like the ones you got at the Citadel) along the perimeter of the circle of runes, with another figure in the center. In addition, there is a Drow (like the ones you fought a few hours ago) lying on its side next to the drider. The drider makes a slight motion, and the drow arches up off the floor, trailing a spectral purple line that connects it to the others and the drider's hand. Its scream tears into your mind as it goes limp, then falls to the floor with a sickening thump. The drider smiles, as it's body and muscles visibly swell at the surge of energy going from the dead Drow along the spectral line to the Drider's hand.

The Drider makes eye contact with your group. Broadcasting mentally to everyone in the vicinity, that same calm voice continues with a confidence and cockiness.
“An even dozen is enough. I need two more of them. You can eat the rest.”

The text in the spoiler is for Hildigrim only.
(As usual, Hildigrim can blurt out something very short as a Free Action, but if he takes the time to explain, that will be his action for the round.)

Roll initiative.

If you roll an 18 or higher, you can post your move / action, otherwise you go after it.

(Drusk rolled an 11, in the Game Log.)

Quillary rolled a 16, but looks up anyway, sure that this is the direction her fate is coming from.

Feldspar rolled a 9, in the game log.

Rolled a 21.

Hildigrim mumbles, "Here goes nothing," grabs the mindflayer amulet around his neck, and reaches for the surface thoughts of the creature.

Detect thoughts - As I understand it, there is no wisdom save for the initial casting and the reading of surface thoughts. It isn't until I try to probe deeper that the save comes into play. Unless the creature has an intelligence of less than 3, I should have no problem reading their surface thoughts.

I'd like to save my bonus on this round until I get the response to my spell (my turn isn't over yet).

Jorin rolled an 8 womp womp

If for some reason the party needs to retreat later, the curve of the tunnel (going from south to westward) is 10' behind the party, just off the the southern edge of the map.

For Hildigrim only:

Hildigrim's eyes widen, and he takes one step back. "Cover! Get cover!" He then turns and dashes back far enough to where he feels he has cover.

OOC: I assume because I move more than 30' away that the spell ends.

Quote from DirtyDogP >>

OOC: I assume because I move more than 30' away that the spell ends.

(The spell is on "self". Every turn for the next minute, Hildigrim has the option of using his Action to re-activate Detect Thoughts on a target within 30'.
Detect Magic expired, because that is also a concentration spell.)

Hildigrim just ducks behind the wall behind the party when the Drider mind blasts everyone in front of it with a cone of mental energy.
Everyone (except Hildigrim) roll an INT saving throw vs. DC 17.
On a fail, take 31 psychic damage, and the character is stunned for a minute.
At the end of each turn, another INT save is rolled to end the condition.

Drusk rolled an 8 in the Game Log. He takes 31 damage (down to 28) and is stunned.

The telepathic voice continues speaking to everything in the cave. "Spellcasters can be annoying. Eat the one that ran away, first."

Everyone (besides Hildigrim) take your turn. The initiative order is: Quillary, Drusk, Feldspar, Jorin

For Quillary only:

Feldspar rolled a 16 in the game log, so she just stands there.

Quillary’s Int Save in game log: 10

So she just stares at the ceiling, stunned. Down to 19 hp.

(OOC: Going to do a little ret-con.
I made some changes compared to a live session from a few years ago, and this does not look promising.
On paper, this thing is CR 9, but when I compare it to things like an Abominable Yeti or an Abjurer, this is just as lethal damage-wise, but then it has that DC 17 stunning on top of everything else. This module has a few more tricks, and I think our party composition will have more troubles than I anticipated.

Jorin still has to roll the INT save, but there are some retroactive changes:

• Save DC is now 16 instead of 17.
• Stun will be until the end of the Big Boss next turn, not one full minute.

Feldspar saves, so not stunned and gets the 31 hp back.
Quillary and Drusk still roll another save at the end of their turns, to see if the stun is still active when the Drider goes again.

I like my encounters to be somewhat frustrating, not dancing on the edge of TPK.
Let's see how it goes with this adjustment.)

(Are you sure? Don’t want you to have to change the module due to some bad rolls, but if you’re giving me my turn I will take it!)

Some questions: squares I assume are five feet, and the drider takes up four squares? What do we see in the middle of the circle of runes? Do we see or hear anything around the rest of the room, like spiders on the walls? Is the drider holding the book or is it next to him? I assume, on the map, the bound prisoners are the lumps on the outside of the ring of runes? Do the humanoid figures bound in ropes seem on the edge of death or perhaps hale and hearty? Would lesser restoration, which takes away the paralyzed condition, also take away the stunned/incapacitated condition, if Feldspar were to cast it?

Quillary: drool, drool.

(Yeah, not the best map (module writer didn’t provide any), but I liked how it looked.

This is what you can tell with a glance. To know more would require your action to be looking over the surroundings, like studying the arcane circle.

The 8 lumps are Alliance scouts, with a more impressively dressed scout in the middle.
They are dirty and really beaten up, barely moving.
The drow who just got zapped is dead-dead.

For a cave, it seems tidy. No plants or wildlife (tiny lizards, ordinary spiders, etc.) that you notice.

Lesser Restoration could remove the stunned condition, and allow a person after you in the initiative order to have a normal round.
If you decide to do that, should probably wait for Jorin to roll his save, in case he is a potential patient.)

Fwiw, for daisy-chaining, Quillary also has Lesser Restoration prepared. But the initiative order is not conducive to this.

Thank you for humoring me - very helpful to get answers!

Good to know, Quillary! Was thinking Drusk would be likely patient but can do her too. I kind of like the idea of all of us quietly restoring each other and pretending to still be stunned at least for a beat.

Was asking about the book in case creating a bonfire under it would be smart - though if it has eyes then it’s lóelelo magical and assumed fireproof.

Will wait for Jorin’s save.

(The book is a macguffin, a prop that does nothing.

But now that you mention it, it would be more fun if it had an actual effect on combat, and getting rid of it would nullify a handicap.
I’ll think about it some more.)

Jorin rolled a sweet 14 and took the full blast in the face.  I keep wanting to use fanatical focus, but i gotta be raging first.