Looking into the “Shaver Mystery” and reading some of the Hollow Earth mythology from the late 19th to mid-20th centuries, these little buggers start turning into some of the bestest antagonists ever. Inhabitants of a mysterious subterranean world of ancient abandoned cities which they have inherited from their own mysterious creators, possessors of mysterious mind-bending rays which can be used not only to torment or control the minds of those on the surface, but actually cause disasters and influence events on a local and global scale. Their penchant for taking slaves from the surface for food, toil, and torture would seem a minor foible in comparison.
This mysterious “ray technology” could be magical in nature, or it could be some sort of technologically-based horror. Personally, I favor the “pneumatic chemistry” of the troubled mind of James Tilly Matthews. I imagine derro outposts nearby to and perhaps beneath major cities, each with its own “air loom” chamber, whence the fiendish savants direct their minions to control events on the surface world to their own unintelligible purposes. Most precious would be the source of the “volatile magnetic fluid” which is used to power the air looms, and perhaps the PCs would be called upon (or take it upon themselves) to staunch the flow of this vital substance to stem the power of the derro.
And imagine the nastiness that could be perpetrated upon the PCs themselves, once the derro were convinced that their plots were exposed and the PCs had become a danger. Using their air-looms, they could not only inflict the unspeakable torments of the lobster-cracking (preventing the blood from circulating) and stomach-skinning, but also sending forth illusions and mental torments to make them appear insane, thus ensuring that those officials who were not already under the power of the derro would not believe their strange paranoid ramblings.
And, naturally, my derro would also be able to take to the sky in their glowing cigar-shaped vessels, emerging from their deep caves to travel at unimaginable speed high in the sky on some mission for their savant masters. Perhaps they, too, operate using the volatile magnetic fluid.
I might also have my derro involved in an ancient and world-shattering war with the vril-ya, who use their own “vril” technology to counter the derro’s own volatile magnetic fluid. And indeed their own flying disks could be set forth to battle the derro’s own flying boats in furtherance of the war effort. But in such a conflict, are the motives of the vril-ya as benign as they may seem at first? Might they not have an ulterior motive of their own?
Can Spring Heeled Jack be far behind, to terrorize the streets of my City of Greyhawk?