When Green Slaad are described as surprisingly intelligent, that is only in relation to its Limbo-spawned relations.
INT save vs. Brin's DC 17: 12
(Mind Whip fail means 12 damage, no Reaction possible, and only one of Action/BA/Move allowed on its next turn.)

Iskander's first strike hits (13 damage), and will keep the slaad from regenerating. The second strike is deflected by its tough hide.

Trimyrai's first arrow is deflected by the priest's Mage Armor. The second hits (13 damage).

Reverie to go. (The priest is still tied and gagged.
The slaad is on its feet, ready to fight, and fuming that its Fireball was cancelled out. It is also really wounded, low on health.)

Reverie will look down on the priest for a moment and then looking back at the Slaad. Soon, she sighs, thinking she should deal with the priest later after slaughter the slaad as soon as possible. So, she will attack on Slaad with her two scimitars.

Attack, Scimitar of Speed - 17 Damage - 11
Attack, Scimitar of Speed - 20 Damage - 8
Bonus Attack, Scimitar - 23 Damage - 5

(Edit: Natural 20!)

Reverie's first strike is stopped by the slaad skin, but the second and third connect.
(All 3 attacks are done with the magic scimitar, and should have the same to-hit and damage modifiers.)
The Limbo creature falls to the ground, and its corpse withering away at a visible rate due to its chaotic make-up. (10+7+critical is enough damage.)

The priest can see that his window of opportunity to escape has already slammed shut. He stops struggling against the ropes.

(Combat over.
You have the time to look around the cave or at the people more thoroughly.
And probably want to roll that rock open so those two in the corpse flower cave can get out of that stench.)

Ana immediately set about checking the prisoners for wounds, specifically and especially the type that slaads make to infect people. She says to the others, "Please help me get these bindings off them." 

Brin will start to look around the chamber as people start to question the priest. He will attempt to move the rock slab aside, to the corpse flower chamber, if he can do so on his own, otherwise will call for someone to aid him in it.

Reverie let out a sigh of relieve. Then she looks at the priests and people for a moment before shifts her gaze towards Ana, realizing she needs help. "I'll help." She says as she walks towards Ana before disconnect the bindings. "Someone needs to help Brin to move the rock and look after the priest."

Without a time crunch, the rock-door can be moved aside with a little patience and effort. The stench coming from that neighboring cavern (and what used to be roving zombies) reminds you why this door was normally closed. The two villagers, still exhausted, manage to crawl out, and nod with satisfaction at the slaad's remains.

Reminder of what was mentioned in post 4744:
The chamber is 50' east-west, and 30' north-south. There are brass cages along the north wall, scarcely larger than a human. There are some low work tables with a number of robes on them. The stone door (leading to the flower chamber) is on the east side. Just before it is a stone slab covered with dried blood, more like a work table than an altar.

The other prisoners are still a bit dazed, but they gesture at the waists of the two acolytes. After some urging from them, you realize they are pointing at a ring of keys on their belts. The keys unlock all the shackles on the 7 prisoners, and allow you to put those shackles on the priest and the unconscious acolyte.

Three of the prisoners are showing symptoms of slaad infection, but you determine that none of them are in immediate jeopardy. As long as you get them treated within the next day or two, they should make a full recovery.

Brin moves to the tables with the robes, searching through the robes, and anything else around the tables.

Reverie looks at the prisoners' gesture towards the acolytes. She says as she takes the keys from the acolytes before looks back at her party. "What do you all think? Should we take the acolytes with us to the outside or leave them behind after asking questions?"

Quote from DragonDenn >>

Brin moves to the tables with the robes, searching through the robes, and anything else around the tables.

The prisoners were doing slave labor on the robes. There is one robe on a stand, an authentic Bane cleric outfit.
The clothes on the table were made to resemble the Bane robe.
(In modern speak, they were making knock-offs that resemble the real thing, but the flaws show when you look closer.)

Brin has more experience as a sorcerer than a cleric, but he (and most of the rest of the party) find it unusual for a "priest" to use Mage Armor and Counterspell.

It is also strange that a god-devil like Bane, who prizes discipline, lawful order, and careful planning, would have his servants working with chaotic creatures like the slaadi.

Isk pokes through the robes with Brin and says, "Cheap copies, seems like this group was trying to impersonate or implicate the Bane Cult with whatever they were doing...putting Slaads into people?  We should take a few of this robes as evidence, along with the priest and acolyte, and hand them over the authorities."

Brin nods his head in agreement. "I too think it a good idea to take these up and show them to the officials in town."

Ana asks, "Are there any papers? Letters, orders, maps and so on? This seems like a part of a larger operation so it would be great if we could get something like that, so we wouldn't have to rely on the cooperation from these cultists."

Trimyrai: (to the party) "Handing them to the authorities might lure out more of them. These cultists might even give up some valuable information after being interrogated."

Reverie nods at Trimyrai's words. "I agree. Also, do you remember how the Bane bribes one of the captains who tried to accuse everything as our fault? During the mission we did for Iron guild? What if there is someone who controls these cultists within the authorities? I cannot think of anything good but the worst." A sigh as she couldn't quite discern which would be the most ideal event.

A pause before she turns to look at Ana and then to Iskander. "Perhaps we can take their robes and search more evidences while we interrogate them here. And then, once we get an information from them..." She hesitates. "... Maybe we could tell what they were doing to these people... To the member of Bane Church."

Brin moves over to the freed prisoners. "Is there anything else down here we should look into?" He asks them. "Is it just these chambers we have seen, or are there other areas, with others of these..." He gestures to the priest and what is left of the frog-creature they destroyed.

(You can decide later if you want to quiz the former prisoners or interrogate the priest + acolyte pair. For now, they trail behind you, and are wearing the knock-off robes to stay warm.)

You return to the 4-way intersection, since you've explored as much as possible from the east and northeast passages. The south passage is where you came in.
That leaves only the west passage.

That corridor is blocked by a Wall of Ice. The ice panels form a "W" across the corridor, so you'd have to break through 4 barriers to continue going west. From previous experience, you know that this was something the bheur hag liked to create. It will take effort to break the wall, and even after the ice is broken the cold air can damage you.

You are able to guess that the spell was cast a few minutes ago. It seems like the hag had the opportunity to join in your fight earlier (with the slaad and priest), and decided to block off this section instead. (One of the drawbacks of working with chaotic evil and chaotic neutral partners.)
The spell lasts 10 minutes, so you can wait for it to come down on its own.

"So anyone have a good story they want to tell while we pass the time?" Brin asks as he nods to the ice wall. "Unless someone has a dispel magic spell in their back pockets..." He glances to the others. "I am also getting low on magic myself, so are we likely to catch this hag now, if we give her ten minutes of a head start?"

Isk looks at his battered body and says, "I am not sure how well I'll fair in another battle, and a Bhuer Hag is no joke.  I'm not normally one to let an enemy go, but perhaps we retire to the surface where we can deliver these prisoners and get some more answers?"

(Isk is at 28/52 hp and has used both spell slots, a short rest would have him back in full form)

(Ana is also hurt and low on spell slots. Not sure we're up for a boss fight. At least lets take a short rest and roll some hit dice.)

Ana says. "We could rest here, but I'm concerned about all these people. And we also need to turn these cultists over to the guard for interrogation. So I actually think we should go back now. The hag may get away but their plans have been ruined for now anyway."