Khessa emerges from her smelly hiding place with great relief: "I don't know... There's something wrong, of course. Why would Vecna ​​reduce one of his own Cultists like this? It doesn't make sense..."

"I want to know if he's under the effect of a spell" the blonde arcane warrior decides "and which one".

The tall adventuress then ritual-casts Identify on the Cultist...

Khessa is able to determine that this cultist has been drained of all vitality and knowledge by a ritual. One could guess a similar fate is what was in store for the prisoners.

"This cultist has been drained of all vitality and knowledge by a ritual!" Khessa explains, horrified and indignant "One could guess a similar fate is what was in store for the prisoners!"

"I just wonder what sense there could be" the blonde arcane warrior is still doubtful "to conduct the ritual on a cultist, when they had so many prisoners at their disposal. Was it a punishment for the cultist? A cultist who wanted to leave or who had discovered a secret that he shouldn't have..." she shudders "Or maybe this one we have before us now is the last prisoner, the one we haven't found yet, who was first reduced to this, then dressed as a cultist?"

"In any case," the tall adventuress concludes, "I suggest locking him in one of the cells where we found the others, then retrieving him when we leave... If it were possible to help him or heal him, not only would we save a person, but also, whether he really is the prisoner or a repentant cultist, we might learn important information."

"That's a pretty big 'if'. Look at her! The poor darling has practically been lobotomized!" His tone is one of mock sympathy. "Besides, regretful or no, she threw her lot in with the cultists. One less fanatic in the world is never a bad thing in my book." He grips the handle of one of his daggers, but doesn't yet draw it. "Perhaps a mercy killing is in order." 

Tristan:

”Being dressed in cultist clothing doesn’t 100% make you a cultist, especially if you are too addled to resist,” says Tristan carefully.  He grabs the poor soul and tosses her into one of the small rooms, then wedges the door shut behind her.

Ozus scoffs, but doesn't argue. Rolling his eyes, he hold up his hands in surrender. "Fine. But you're babysitting her on the way out. I'll not be slowed down by the poor decisions of others."

Khessa tries to bring the focus back to exploration: "Well, nothing is slowing us down anymore for now. So... do our scouts want to explore along the corridor the cultist was coming from?"

The blonde arcane warrior expects to find something interesting there... perhaps the laboratory or temple where it was possible to perform the perverse ritual that completely messed up the victim's soul.

The corridor stretches south, with several narrow doorways leading to empty crypts. There's a door to the south and to the east.

Khessa grants herself a minimal magical protection, then attempts to eavesdrop, first at the door to the east, then at the one to the south.

@DM: I don't know if you prefer one roll or two in this case... I'll roll two (the first for east, the second for south) - if you prefer to use only one, let's use the first of the two!

Perception 1: 12
Perception 2: 26

While Khessa does the responsible thing, Ozus is snooping through the various crypts, hoping to find anything worth pilfering.

Ozus doesn't find anything at all in the crypts, they've been cleaned out pretty well.

Khessa listens at the eastern door...but it's quite thick, and she can't hear much of anything beyond it. At the southern door, however, she quite clearly hears a group of people chanting in a strange language. Their voices echo, as if the room is rather larger than the ones you've seen so far.

Tristan:

Tristan returns to the rear of the party to watch their 6 and protect the rescued hostages.

Purely out of curiosity, Ozus is going to have a listen at the southern door. Does he recognize the language?

That's a 21 for perception

Naero 

Waiting near the rescued hostages while Tristan had been near the front, he relinquishes his defensive position when the knight returns. He moves forward. "I say we clear the quiet room prior to entering that one" he whispers, pointing to the southern door. "I can return to animal form once more, but that will be all if we have nowhere to rest. I think we can handle whatever situation resides on the other side of that door but I will scout under the door if you all wish" he offers his natural talent to the group.

Khessa suggests: “I was thinking of checking this out, too first” and jerks her thumb toward the silent door. “But I’d save your talents for later, Naero, just in case we find a place with more options to examine. This door… I say we just open it and see what’s inside.”

"We could also just call it a day and flood the place. That's what those pipes near the entrance were for right? Besides, we have what we came for."

"I'd so love to!" Khessa sighs "But we still have one hostage to recover: Eldon Keyword, the highly respected, and highly knowledgeable, scholar specialized in the Outer Planes"

(So are we opening the door?)

(Ozus, give me a religion check)

That's a 14, not too bad. Ozus knows infernal, abyssal, and undercommon