The Plague House:

Status: No longer in use.

Immediate danger: The entire dungeon is slowly sinking. Every once in a while, roll a d6, 1 means it sinks further.

Who or what dwells here?: A meaty mass of slime, larvae and spider legs.

What brings you here?: Rumors of occult treasures that might alter the world's fate.

Entrance: The gaping mouth of a church-sized glass skull.

Guarded: A tangle of withered tendrils. They come alive!

Distinctive feature: Obelisk that separates body and soul.

Room 1: A broken altar. Mirrors everywhere. Obelisk stands here.

Room 2: Compact darkness. A drum.

Room 3: Spider webs cling to the wall. A puddle with acidic mud. Within it, a key.

Room 4: Blood-covered beds. A random sacred scroll is hidden under a sheet. In a corner is a big spider. A person sits on one of the beds.

What I made of it:

The treasure is an amulet that has many rumors surrounding it.
Some say that it changes the wearer into an omnipotent being; some say that it can stop the end of the world.
The reality is the wearer must make a DR 16 Toughness check the moment the amulet is put on.

If they succeed, they become the lord of the dungeon, able to command it to do anything they ask of it. In addition, they get access to a random unclean scroll; the first use is always successful, and no roll is needed.
In exchange, they can’t leave it anymore. The exit becomes invisible, and if someone else leads them out, their head explodes.
If they fail the check: They become a meaty mass of flesh and spider legs.

Entrance:
Once the players get close to the dungeon, the tendrils barrier the entrance. Cannot be defeated in a fight. Progress by high presence check to persuade them or usage of light.

Room 1:
Attacking the mirrors reflects the damage back to the attacker.
If they touch the obelisk, their souls separate from their bodies, allowing them to pass through a mirror. There, the same room awaits, but the altar is impeccable, and a meaty mass starts to push through. the floor.
On it is a depiction of someone smashing a drum.
They can return to their body by simply passing back into it. If the soul leaves the body for too long, the body becomes a part of the meaty mass.
The meaty mass starts to appear once mirrors fall down because the dungeon sinks. It will always barricade the exit on the players' way out.

Fleshlumb Statblock:
Initiative: 1-2 on d6, Armor Tier 1 Hard skin (-d2), 40 HP,
1d4 damage and a DR 12 Toughness check. If failed, the target has to skip its next turn.

Room 2:
Within the drum is a random occult treasure. If the players try to play the drum, it spawns 1d4 Lady Porcelain (refer to page 66 in the rulebook) enemies.

Room 3:
The puddle inflicts 1d4 damage. A torch can dry it up. If a player tries to reach in, there is a 1d4 chance that the puddle forms a hand and reaches for them.
The key in it is for the final treasure chest with the amulet.

Room 4:
The person within is trapped in the room. They can’t see any exit, and if the players try to lead them out, their head explodes. There is a random treasure in the remains of the head (refer to the corpse plundering table).
The spider in the room shares the stat block of the random enemy that might appear in the Rotblack Sludge one-shot of the rulebook. It is the person's pet spider called Susi. Neither the person nor Susi is aggressive if unprovoked.

Room 5:
A treasure chest with the amulet. The key from room 3 has to be used to open it.

My experience running it:

I run it once with a group I sincerely love. There is a saying that to calculate the IQ of a TTRPG party, you simply take the player's highest IQ and divide it by the number of players. As a player, I am not immune to that effect.
One of them rolled a player character that had one HP and a bomb. Another rolled a Gutterborn scum with the ability to spit. The other characters I do not remember.

The entrance was not solved by them, and this was my fault: I planned a riddle where they had to search for a lens, but I did not explain that very well. So the Gutterborn scum spat on the pedestal meant for the lens. I ruled this as fine, since I guess that reflects light as well, and I did not want them to get frustrated at the entrance. Ditch the riddle. If you want to run the dungeon, I suggest making them aware that the plants are sensitive to light instead.

Room 1 went nothing like expected. The players ignored the obelisk completely and just set the door to the next room on fire. It was not locked; they just did not check before getting the torches out. So the darkness of the second room was gone as well. Fantastic.

Room 2 was now just a small room with a drum. As I hoped, a player, the one with one HP, immediately began to play it, and the d4 decided to spawn 4 Lady Porcelains. If you know these enemies, you know that four of them are a bit of a problem. Combat ensued, and the very same player decided to play the drum again. Therefore, more enemies.
However, that was fine. The bomb-bearing glass bones told every other player to get out and miraculously survived one round to use the bomb. A glorious end for the player character who seemed to be rolled exactly for this maneuver.
Within the same room, I decided to spawn their new character so the player could keep playing without interruption. Funnily enough, the character had the quirk of talking to a skull, so their old character never left the party, in a way.

Room 3 was uneventful. They used a torch to dry the mud and got the key without any problems. And then immediately forgot about it when they realized that they did not need it to open the door to the next room.

Room 4 might have been my favorite. Players ignored the person and headed straight for the spider, Susi. Presence checks were made, and they ended up adopting her.
Afterwards, they talked to the person who sat on a bed, confused. They discovered they could not leave the room or even consider the possibility that there might be more beyond it. When they tried to lead them out, the head exploded. One player promptly asked me if she could loot the body. I let her roll for it, and she got a natural 20. So I decided to hide an occult treasure in the remains of the head on the fly. Otherwise, it would have been something from the plundering table.

The treasure room was lovely. Players stood before the chest, attacking. I took pity on them and gave them the hint that they still had a key.
At that point, I want to reiterate: I think that they are smart and capable people. Each and every one of them is a person I genuinely respect and cherish. That being said:
Amulet in hand, they took 10 minutes holding it, trying to find some magic words to activate it, rotating it, and throwing it around before they asked to make a presence check to find something to activate the amulet. Had to tell them that they could put it on.
The player who previously played the 1 HP bomber decided to put it on and actually beat the DR. So I told them that their body became that of a lich and that they felt connected to the dungeon now. Sadly, they did not try to test this.
On the way back, the meaty mass blocked the exit. The player with the amulet was able to command it to let them through, but they themself could not perceive the exit. Seeing the pattern recognition, they still asked the other players to lead them out.
When they asked if their head explodes, I decided to fake them out and told them no, but their body does.
Another player jumped in and asked, "But you said the character became a lich. Does that mean their head stays alive?” You know what? Sure. He had a valid point.
The character who wielded the amulet became a talking head that the others placed on Susi, the spider.

To my dear players: Thank you for playing with me. It was a memorable round and a great noon. Your antics are a big reason why I enjoy Mörk Borg as much as I do.