As Rhogar and Krom study the altar they feel warmth being sucked from them. Something is drawing heat from.  

Chill, though not affected by whatever is affecting Krom and Rhogar, does notice that the room with the altar is considerably colder than the other two rooms and as he has moved between them there is a noticeable temperature drop in the room with the altar.

Chill's search of the desk only revealed a few sheets of paper, with nothing of any importance in their contents.  However one of them does bear a symbol and letterhead of the Arcane Brotherhood.  The same Arcane Brotherhood Vellynne belongs to, as well as the prisoner in Revel's End, Vaelish Gant.

"Mn. I'm certain now more than ever. You felt it too, yes? The room drop a few degrees colder?" Rhogar asks Krom around chattering teeth. Though her hardly waited for answer before taking the dwarf's shoulder and urging him to back away along with him. "Wide breadth, or at least wider one we should give this one, least whatever power this altar holds freezes us to death. I'd even suggest destroying it, but... not yet. Not until we've secured the prisoners and everything else is settled." He'd further insist.

But, if his more deity devoted companions insist upon its immediate destruction, then... Rhogar doesn't stop them. Instead regardless, unless they request his aid specifically, he'd finally take Chill's request into mind and check out the other room with the lever. And if upon seeing no signs of markings anywhere near suggesting what it might be fore, he'd warn, "I'm going to try something that I may regret. But out of all of us, I think I have the best chances of mitigating any dangers to myself. So keep to the halls or other chambers for now. Also, keep a hold of those other items for now. Needless to say, I'd wager they are of magical variety, BUT will require a bit more study than the moment provides." He waits patiently in case any had followed him into the chamber to depart before grasping the lever, taking a breath, and then pulling it...

-Perception: 22.

"I agree, let's worry about this altar after freeing the prisoners." says Krom while giving a wide breath to the unholy place. "It may be necessary to temporary dispel the magic around it before attempting to destroy it. Tomorrow I'll attempt to commune with the All-Father for his advice."

Noticing the dragonborn's intention with the lever, the dwarf chants an hymn of guidance while touching Rhogar's shoulder. Krom then follows the dragonborn's advice and retreats in a nearby chamber. He does his best to keep Rhogar within his line of sight.

(Sorry for the delay guys.)

(@DM is Kazri able to see what’s towards the eastern end of the hallway. The double door.)

Kazri, from her spot at the entrance to the southern corridor where the others are, can see the slab that covers the eastern end of the cistern.  In the dimlight she sees that right after Rhogar headed into the room with the lever, and presumably had pulled the lever, the slab moves.

Rhogar can hear, from where he pulled the lever, a low rumbling.  Something happened.

Kazri can see that the cistern continues past the opening but the range of her light falls into darkness and she cannot see further in.

Chill and Kazri have used about 5 minutes of the fly spell

"Mr. Nimbatuul. Keep pulling the lever, whichever you pulled. I will investigate the eastern end of the cistern." Kazri takes out a torch, lights it up, then she will fly to the east. She lands on top of the the slab that covers the cistern.

(Not sure if it's only me, but I am not able to see the map above.)

(@Wolfy: Might just be you, as I see it.)

"I will hold it then for the moment!" Rhogar calls back, further affirming his grip on the lever. "Chill. If you would be so kind, do go with her as well. Just in case there are more demonic guards in the new area." He further request. Following any sounds of affirmation of the request, he sighs softly and chuckles inwardly to himself. "If my brothers could see me these days." He quietly jests.

"Chill, I'll stay here with Rhogar until the passage is wide enough for our boat. Kazri has keen eyes, but you may spot things she may not notice." says the dwarf to the tabaxi as Kazri informs the group of the effect of Rhogar's action.
"The enchantment that Rhogar has placed on you two is not going to last too much longer. Two pairs of eyes will make things easier."

(@Wolfy: I see it too)

"Sounds good.  But there better be a place to land.  I hate getting wet."

Chill heads of with Kazri east deeper into the cistern.

(I see the picture now. I think it was my spotty Internet connection.)

(So both Chill and Kazri fly to the cistern.)

Rhogar heard the sound of rumbling and knew the lever had done something.  Confirmation came from Kazri.  But he found it wasn't necessary to hold it.  Pulling it locked it in place.

Kazri, with the aid of the lighted stone Rhogar had given her, was accompanied by Chill as the two if them floated over the water deeper into the cistern.

The dark cistern continues for another 40 feet beyond the marble slab. The water in this part of the cistern is thankfully free of half-frozen corpses. A narrow tunnel carved into the north wall provides a place to land. A five-foot-wide, eight-foot-high tunnel leads to a dark, cold room that reeks of filth.

Though a touch reticent to let go of the lever, the dragonborn yet warily allows his hand drift away, only to stare at the lever for close to a minute. Nothing happens. He smirks and snorts and soon after departs shaking his head over his own paranoia. "Seems I was wrong about the lever, Master Krom. Shall we return to the boat and try to catch up to others? After all, -if- there are yet any prisoners remaining, they'll need some sort of transportation. And while I have no doubt Lady kazri can ferry a few across herself, my spell will not last forever, and I have not the magic to recast it with as much 'oomph' as the current one." Rhogar confesses to the dwarf.

"Yes, Rhogar, let's rejoin the others." nods the dwarf as he follows the dragonborn back to the boat. "I may be able to craft us a small bridge of ice or maybe even another small boat by chanting one of my most powerful hymns of creations in case we need another way to cross the cistern." adds Krom as he climbs on the boat.

Chill peered into the room from the spot where they landed near the edge of the water.  He motioned for Kazri to wait a moment as he also listened carefully.

Perception 28

He then turns to Kazri.

"So, do we declare ourselves?  We seem to be at the end of the cistern.   If the prisoners are anywhere they would be here."

"Agreed Chill." Kazri steps one step ahead of Chill and yells. "Hello! Anyone here? If there is anyone hiding here. You are safe now. You can come out. No harm will come to you..." Kazri's voice trails, as she is unsure if she is actually speaking with anyone. Using the lighted stone, Kazri tries to look into the room.

Perception: 7.

Both Kazri and Chill pause and look and listen. A stench wafts from the room and initially they cannot see anything.  After Kazri calls out though they can hear whispering but cannot make out what is being said.  They also hear shuffling as people move.  A moment later a voice calls out. "Help us please."

A older man, looks to be in his sixties, steps into the light.  "Who are you? Please help us." 

“Does anyone needs healing?” Kazri speaks towards the older man.

At the word of healing a couple other people come out from behind the wall and into the light.  A older tall thin man and a slightly younger stout woman step out. "Please free us.  We haven't eaten in 2 days.  We have no injuries but hunger and poor treatment."

You are certain there are a few more behind the walls as the 3 in the light seem to look back at others out of sight. You hear the stout woman say "Its OK, its not them, I think they are here to help."

"We are here to help.  We are not the cultists and have captured or killed many.  We are here to set things right.  We are the Fellowship of the White Cloaks."

Chill stepped forward with this hands open and then brought out rations to give the people if they accept them.

Kazri leave her shield propped by the entrance, and goes to help as best as she can. She takes out rations from her backpack and gives to the older man and woman. “How can we help you?’