Location: Dripstone Cave

Taking a brief break from NPCs. Unfortunately I forgot to save the name of the Redditor that provided this image, so I do apologize for that. The inspiration for this post came from the picture, but also from Amnesia: The Dark Descent. It's pretty heavily altered from that, but The Shadow was the theme I was going for behind the goo. Also, someone pointed out that my previous font size was too small. So hopefully this is easier to read without being too obtrusive.


Description:
The cave seems fairly normal at first, twisting and turning, opening into large passages like most caves do. Deeper in, however, you see a strange fluorescent light that illuminates the cavern. You see a viscous liquid pooling on the floor, emanating an eerie yellow glow, and fills the rest of the cave. The stone is coated in a tough shell that reflects the light. The liquid bubbles slowly, and you can feel it writhing around your feet very slowly. It smells of rotted corpses, and the color shifts to every imaginable hue as you progress.

Associated Knowledges:
Arcana:

• The goo has a strong magical aura about it.
• You can detect that the goo is self-propagating.
• The magical trail leads to the depths of the cave, intensifying the deeper you go.

Nature:

• The cave itself is of natural origins.
• The goo is by no means natural.
• The cave itself is being irreparably altered by the goo.

DM Info:
  Dripstone Cave was, at one point, just an ordinary cave. Now, however, it has been filled with a mysterious goo that is slowly eating away at the stone, and turning it into something else. A planar rift formed in the depths of the cave when a demon attempted to enter the material plane, and now chaos is warping the cave into something from it's home plane. The goo is slowly creeping up towards the surface, but the process is very slow and difficult, as the demon failed to enter and is trapped in limbo between the two planes.
  The goo itself is not sentient, but it is capable of some level of thought. It has the overpowering urge to creep up towards the surface. The goo is eating the stone of the cave, but not in the way that most things "eat" their food. By consuming the stone, the goo becomes more grounded - pun fully intended - in the material plane. The essence of the material plane is shifted into the goo, and the stone is replaced with a replica from the demon's home dimension. The goo isn't necessarily aware of what is going on, as this is more of a reaction than an intention. The demon will be able to fully enter the material plane once it has established a domain that is akin to it's own home plane, which the goo is accomplishing.
  The creatures that lived in the cave have either died, or undergone mutations from the goo. Some of them were outright eaten, and their petrified remains resemble the stone that has been altered. Others were exposed to the ambient magic energy, and transformed into aberrations. Mostly they wander aimlessly, as they have no comprehension of what has happened. Some smaller monsters from the demon's home plane have also come through the bridge between them, and now inhabit Dripstone Cave.
  The environment inside the cave is inhospitable to life of the material plane. Toxic gas and dangerous energy radiates from the goo, and closer to the rift the magical energy is highly unstable. This requires magic or other methods to make it even possible to enter. The goo itself slowly tries to "eat" the PCs if they step into it, but the process takes so long that it should pose no direct danger. If they stand still or go to sleep in it, however, they could never wake up at all.
  The challenge to PCs adventuring here would not be direct combat. The creatures that live in Dripstone Cave are weak enough to slip through the cracks between the planes, or mindless animals that were altered.The environmental hazards are enough to challenge PCs. The goo has flooded some areas entirely, and the deeper they progress into the cave, the weirder things get. The stones start growing eyes, random chaotic traps appear, and dimensions make less and less sense as reality gets warped. At the end of the cave, the demon still hasn't fully formed yet, and the combat should be relatively easy.