Describe the creature(s) who are hiding amongst the inhabitants.

The mountain folk of Scrim live unaware that many of their kin no longer live among them. In their place the Fetch carry out the orders of their Fae creators, building the architecture of the next Riding.
When a Faerie or their agent steals a human they take a piece of it, a bit of shadow, a reflection from their eye, their sense of truth or lies, their first kiss and first crime, or whatever else strike them as potent and expedient. The rest is built from shreds and scraps and detritus, or unwary family pets. The magic of the Fae hides them, makes them seem of flesh and life. Many of the Fetchfolk are the abandoned tokens of the Lords and Ladies, unknowing replacements and as much victims as the humans they duplicate, but a few act as their agents. That few is enough to be a threat.
The Fetchfolk are not Changelings, though they share the glamour-wrapping which gives them the appearance of humanity. A Changeling is an aging faerie, pawned off on a human couple as a parasitic hospice. The Lords and Ladies are callous creatures, and do not care for their infirm. Their cruelty is even more obvious when one considers that their kind are naturally immortal; Age is a function of psychological health and social station for these beings, not the inevitable passage of time. No Faerie dies without someone being to blame for it, and typically it's one of their own.
The Lords and Ladies themselves are not yet the denizens of this world, but they seek it. These are they that circle the candle flame of this reality, looking for a way to pass inside. The cold stability of this world will burn them, and their passing into it may snuff it out, but they can no more resist its light than a moth. Everything that holds together in the lands beyond Creation is cast there by the light of this realm. Without our dreams there is no common language of thought for the Fair Folk, no point of reference to build their dream-bodies, their miracle-markets, their nightmare-art.