Probably the oldest of them. Long before even Charles de Lint there was slick fantasy -- that is, fantasy published in slick magazines, as opposed to the types published in the pulps. Whereas the pulps were creating sword and sorcery, the slick magazines would have things like the deal with a devil, or the three wishes, or a little shop that wasn't there yesterday in the modern world.
And why was it secret? Was some invisible force compelling them to keep it secret for mysterious reasons?
Hard to tell. Because in those tales, the characters in the masquerade who actually appeared did nto reveal their motives. Might have been forced, might have been free, and for any number of motives. They weren't talking.
Restricts what tales you can tell, but what justification doesn't?