Most of the prisons have skeletal remains on the ground of the long dead prisoners and just the shackles are hanging from the wall except for one cell. There is a human skeleton clad in a tattered black robe.

Lando shakes his head in disgust. "Poor soul, chained to a wall is no way to die"

He attempts to free the skeleton from the shackles on the wall.

If the hands aren't skinny enough by now to slide them through, he'll break the hands off first. Once freed, he lays the skeletal remains on the ground, and offers a quiet prayer to Jergal.

Tilo grimaces in distaste as he looks around the room. He points out the hidden door to the others when he spots it.

As Lando breaks the hand free from the shackles you notice a gold ring on its finger as you lay the body down and say a prayer to Jergal. The chanting is louder hear in the prisons coming through the east wall. 

Noticing the ring, "Aren't you a dear thing" he thinks to himself, and slides it off the finger.

He holds it up to inspect it, then places it in his pocket for safe keeping, not trying to hide his action.

He gives the skeleton a quick nod, then joins Tilo and the others.

Reggie waits with Tilo near the hidden door. He leans against a wall with his eyes tightly shut. When he hears Lando approach, he opens his eyes and nods for Tilo to proceed.

Andrei peers left and right, trying to see if there's anything coming, before he waits for Tilo.21

Perception: 

The chanting stops as you peer into this forty-foot-square room. The smooth masonry walls provide excellent acoustics. Featureless stone pillars support the ceiling, and a breach in the south wall leads to a dark cave heaped with refuse. Murky water covers most of the floor. Stairs lead up to dry stone ledges that hug the walls. In the middle of the room, more stairs rise to form an octagonal dais that also rises above the water. Rusty chains with shackles dangle from the ceiling directly above a stone altar mounted on the dais. The altar is carved with hideous depictions of grasping ghouls and is stained with dry blood.

Half embedded in the northern wall is a wooden wheel that you assume will open the portcullis.

Andrei grimaces as he stares at the dais, trying to figure out what the apparent sacrifices on it would be used for. He holds his ice ax ready.

Arcana: 18

"What's going on?" Reggie asks from the back of the line (still in the prison). "Why did the chanting stop?"

From where you are standing in the prison, Andrei has no idea what could've happened on the Dias. 

Lando steps into the large room, and moves to the side so to not block the entrance way.

"By the gods..." He appears speechless for a moment.

"This looks to be some sort of sacrificial alter. I bet the prison was just a holding room for their ceremonies." Lando spits on the ground. "Disgusting"

Noticing that the chanting as stopped, he grows suspicious.

He closes his eyes for a moment then says,  "Jergal, reveal these creatures who refuse to accept their true fate!" A faint bluish aura seems to radiate from Lando momentarily and then is gone.

As Lando opens his sight to see any of the undead that may be in hiding his senses see nothing at the moment.

"The coast appears to be clear... for now"

Lando motions for everyone to join him on the ledge.

Reggie walks to the ledge and looks down into the water.

Perception: 19

The water is 2 feet deep. The ledges and central dais are 5 feet high (3 feet higher than the water’s surface), and the chamber’s ceiling is 16 feet high (11 feet above the dais and ledges). The chains dangling from the ceiling are 8 feet long; the cultists would shackle prisoners to the chains, dangle them above the altar, cut them open with knives, and allow the altar to be bathed in blood.

Under the water you don't notice anything dangerous. 

Reggie grunts and walks south and around the corner to look at the pile of debris.

"Well, looks like the only exit is through that gate." As he says that he finds a heavy loose piece of masonry or similar object and throws it into the water where the wheel to open the portcullis is, watching for a reaction under the water.

Perception 18

Artus will look around. "Yes, that maybe the way out, but the chanting has stopped... I have a bad feeling about this..."

Andrei walks over to examine the debris pile, too.

"I don't like this. Why would they put their junk in here, where they did sacrifices and rituals?" he asks Reggie.