Dero

 Have you ever felt a need to run a stereotypical little gray men from UFOs story in TTRPG form, but decided it was ill-fitting for your fantasy setting? Worry not, the dero are here to save you. 

A band of warriors wakes up in a dark and moist dungeon, shackled to the walls. The night before, they had celebrated a major victory against the invading forces of Niblingast, and they must have gotten very, very drunk - as all of their memories appear very hazy, and they haven't the faintest clue where they could be.
Soon enough, a group of small, milky-eyed humanoids enter the room without a word. They feed their captives some sort of fungal sludge, and one of them slips several keys in Fluki the Shade's hands, whispering in a broken form of elvish: "if you survive what is coming, seek me out. I have an interesting proposition".
After the first group is finished, another enters - this one accompanied by a cave giant, and led by a cackling, syringe wearing elder of the race.

Synn, an aasimar who can trace her lineage to the actual Sun-god Zhaa, is approached by a strange man in a long black trenchcoat. The "man" turns out to be two dero standing on top of one another. The uncanny humanoids have a proposal: If Synn is to allow them to clone her - and "care not if the clones get murdered, hurt, and/or eaten in the pursuit of enlightened science", she will be given access to their vast amounts of research on Zhaa. Such knowledge could unlock her true angelic potential, but it also hides secrets best left undisturbed...

The proud city of Arkhangast is falling into darkness. Literally. Over the last few months, entire households have started collapsing into the ground overnight, as their foundations mysteriously vanish. The city was built on top of a large complex of deep gnome tunnels - but those have long since been purged. A deeper (pun intended) investigation reveals that the tunnels have been repopulated - but not with the svirfneblin but with previously unseen humanoids, the sun-fearing dero. The paranoid little humanoids gather around a messiah whose ultimate goal is as simple as it is insane - if the dero can not rule the Overworld by inhabiting it, then they shall simply bring all of the overworlders into the blessed Underworld, even if some (or most) of them do not survive the trip.