Stygian Oil is a liquid with magical properties that can infect an animal and control it against its will.
There are ruins in Vosca that date far back beyond recorded history. Even the Elves, for the most part, cannot reach back far enough in their cultural (or at times individual) memories to recall when certain things were made.
The Black Spires are one such phenomenon. A series of great, basalt obelisks that stretch a thousand yards into the sky, and are surrounded by fields much smaller, angular constructions, and a floor of similar stone so dark that on a hot day, flesh will burn, bubble and peel at the slightest contact.
These spires are ancient beyond reckoning, yet there is no race yet that has dared to approach one that has not found at least one language etched onto its great surfaces that did not bear enough similarity to one of their own languages to be translated, or, in rare cases, to simply be their language in archaic form.
Much of the words have been lost to time, but some surviving snippets say things like "We are mighty, it is important that you know this," and "This marker is one of many. It celebrates nothing. It is a warning. You should leave. There is nothing beautiful here."
Obviously, these markers are the source of intense scrutiny, interest, and investigation by those cultures that discover them. Most have tried, in some way or another at some time, to penetrate their defenses and find if any hidden treasure, or additional information, is hidden within. So far, no treasure has been found.
But what most don't know — can't know — is that however far the spires extend upwards, they extend further the other way. Deep in the dark heart of the world, the spires guard an ancient, dark secret.
And those rare few attempts that have penetrated the Spires into the depths have uncovered a horror that is as likely to be the final line of defense of the dark secret as the secret itself: The Stygian Oil.
In its natural state, it is a slightly viscous liquid. It does not react unless disturbed, by digging, frequent investigation, or contact with any sort of light.
When aggravated, the Stygian Oil begins to seep out from its pools, grasping blindly for something living. Whatever it finds, it insinuates itself into, entering through pores, orifices, or by creating chemical reactions, and melting away at the surface until it reaches organic matter.
Once inside, the Oil spreads, and starts changing things. With access to complex organs, it can now see, smell, hear, and operate further away from its pools. It can twist the flesh of living things to suit its purposes—even to the point of melding multiple creatures together in the pursuit.
The creatures possessed are not killed, immediately. They are quite awake for the entire process, and can at times struggle to regain control of themselves... or parts of themselves. The most common is the mouth, so that they can scream.
Once disturbed, the Oil's only real motive seems to be to kill every living thing within about a mile of its resting place (though this radius expands depending on the gravity of the disturbance) after which point it drags its possessed forms back to its pools, to disappear into the bottomless pits, and never be seen again.
The Goblins have discovered ways to fight this abominable stuff, but more often they hide, or leave, rather than confront it once it is awakened, usually by foolish outsiders.
Most of the obelisks near human settlements have been walled off by now – the Ruvians famously built walls around the ones they found and then filled them with cement, creating large concrete cylinders which still stand to this day. If the Stygian Oil opposed this measure it did so unsuccessfully. Many of the obelisks still stand out of the wild, however, some even sitting on small islands in the Shattered Sea and beyond. The exact purpose of the obelisks, who built them, and what the oil is, are all questions that baffle scholars.
Stories abound of people who have made it inside or to the obelisks (depending on their size) and charcoal rubbings of the supposed writings on them are for sail on every street corner... But it's hard to tell what is authentic and what isn't.
A common theme of all the stories though is that there is never anything valuable inside. No gold, no jewels, no neck-torques, no troves of silver. Just black basalt walls, and etchings.
The Black Spires are based on the US Department of Energy's plan to store nuclear waste so that future generations will stay away from it.