“Oh by all that’s holy, what is that smell?” Feris choked out.
The corridor branched off to the left. A thin trickle of sludge flowed from the side passage into a small grate in the middle of the intersection. Nimble squatted down, sniffed, dipped his finger in the sludge and held it close to his nose. “Smells like... like... street food at the end of a long day...”
"It's worse than that." Rathgar gagged.
"Oh come on... we fought that excremental a while back. That was way worse."
"I have to agree with Nimble." Allianora shrugged. "We've smelled worse."
"Yeah, but this smells like bad food."
The sound of something heavy and wet slapping the ground reverberated down the side passage, followed by a squelching that sounded like it was getting closer. The pinkish grey sludge got thicker, and more of it flowed as the sound got louder, until they could see it. The blob of fat, shit, hair, and trash flopped toward them.
Armor Class: 9
Hit Dice: 6 (L)
Move: 45’ (15’)
No. Appearing: 1 (0)
Save As: F6
Treasure Type: B
Monster Type: Lowlife
This giant ooze builds up over years, accumulating mass from the castoffs of civilization. As such it also tends to accumulate items of value within itself, and anyone who manages to slay it, and is willing to dig through it's remains (a process which takes 1d4*6 hours of effort) can collect the treasures. The fatberg attacks by rolling into the nearest creatures. It can attack up to two adjacent creatures at a time. It will only take 1 point of damage from non-magical weapons, and 1+ whatever magic bonus magical weapons possess. It takes full damage from spells, and fire damage causes it to combust on a failed save vs dragons breath, for 2d6 points of damage every round until it's dead. However, while burning it will spray boiling fat out 20' in all directions causing 2d4 points of damage to anyone in the area of effect.
Based on the 15 ton blob of fat found in London's sewer.