At the bottom of the stairs, the party steps into water. The tainted water is 2 feet deep and seems to be the source of the smell. The room is unlit with the exception of both your lamps. The room looks to be hewn into the stone here as the only supports are the wood beams along the ceiling. A corridor goes on but turns a corner, the party is unable to see beyond that.

(Will clear the grid coords for exploration. Will focus on a room when entering into combat along with giving it coords.)

Zim follows the walls walking counter-clockwise. From f1 to g2, f3, e4, down to e7 to look around (perception 18)

Arkon will keep watch over the hallway leading to the turn, ensuring nothing can sneak up on them from that way while the others look around this section.

Perception: 13

The corridor continues on and Zim sees it does another turn. The only difference is that the corridor goes up a bit, stepping out of the sewage.

Valeria is ready to follow, letting the dwarf make the decisions on which part of the lair to cleanse of foul cultist first, and while waiting she makes generous use of her magic to create a more pleasant scent around her to counter the stench around her. Whenever Zim decides a direction for the team of Flaming Fist deputees she would again send Mal ahead to scout for threats.

Arkon follows behind Zim, disappointed that no enemies have shown up yet.

Quote from Virst >>

The corridor continues on and Zim sees it does another turn. The only difference is that the corridor goes up a bit, stepping out of the sewage.

Put your spoiler here.

Zim returns to the group and says, "Get ready, we're going in." And he leads the team to the bend (L7).

Sarris will move over to A1 to peer down that length of 'hallway'.   First using his darkvision and then, if unable to note anything other than stone walls, perhaps moving to A4 and holding forth that skull lantern to illuminate what is 'below'.

Passive Perception: 17

For advancing party -
Zim comes to the corner and he can see a door at the end of the corridor. Carved into the wood of the door is a relief.

For those still at the first room -
Sarris does not see anything through the tainted water but does note a similar indent in the wall at the other end of the room.

Dungeon Map -

Gorin follows closely behind Zim, and close to Arkon and his lantern.

"Well that can't mean anything good" he says upon seeing the relief. He than looks at Arkon "Wouldn't you want to have your hands free for when we encounter any of the cult? I can hold the lantern and stay close to you if you want."

Arkon nods to Gorin. "That would be good. Thanks." He gives a slight smile and passes the lantern over, drawing his flail and shield instead. The picture on the door is a bit creepy, but still just a picture so he isn't too concerned.

The redhead looks behind her, hoping that the others would catch up so that they were in formation when they ran into more cultists, splitting up and being picked off by hidden assassins wasn't a part of the plan.

Zim calls back to the stragglers, "It's now or never, I'm going in..."

As Zim walks down the "tiled floor" of this new hallway, he makes note of the iconography carved in the door, he recognizes Baal but gives the image no mind. Zim opens the Door!

Stealth 12
Perception 15

The door opens with no difficulty and leads to a small room. To the west of the room, it goes into a corridor. While the east and south are both with doors, similar to the one you opened.
Eastern door -

Southern door -

Dungeon map - 

Sarris takes note of the odd depression in the wall but hears his companions working their way down the other hallway.   Moving swiftly but with as much stealth as slogging through a cesspool allows, he will move to hold the rear position as he did before.

Zim signals to the other, "I know that guy too, it's Myrdul! I hate that guy."

Zim moves at half speed, casually sneaking through the octagonal room to open the Myrkul door.

"Seems like we should follow the bad pictures to find the bad guys." Arkon observes with a shrug, following Zim though stealth is a bit of a lost cause at his size.

Perception: 9

Stealth: 2 (without disadvantage would've been a 21 lol)

The door opens and the room holds some light from the torch in the center. Yet it just shows a grisly scene of blood patterned on the floor with three corpses surrounding it. The corpses were dressed in black robes, with their hands outstretched towards the fire. Blood seems to drip from their wrists.

Dungeon map -

Arkon steps back after seeing into the Myrkul room, not out of horror but to casually wave Val over. "Looks like magic. Kind of devilish. Can you tell what it does?"

Valeria comes up behind the massive goliath, peeking into the room from behind him, trying to recall anything useful about what she sees in there. "Quite cosy, reminds me of my girl's room..." She says with a wink to Zim and Arkon. "...just kidding, I had way more corpses laying around." She adds with a grin, hoping they all realized she was kidding. Their home was quite normal among patriars and the only thing to give away they had any dealings with devils was some very particular contracts sealed away safely from prying eyes.

(Arcana/History: 16)

"You know these bodies are likely undead. Perhaps giving their skulls a good bash will make sure they won't inconvenince us." She suggests to Zim and Arkon, seemingly having no qualms over disturbing the dead. After what happened up in the baths in painful memory, the redhead directs her invisible imp to hover just above the farthest corpse to give it a stab, just to make sure.

(If relevant, Stinger: 24 Piercing: 9 Poison: 7 )