The seraph cities

Before the Splinter War, the seraphs lived within the community, a flying people known for their crafts, and to best knowledge, shared their floatstone cities.
However, in the War, Nirill fell, and when Minaaré was invaded shortly after, the attackers were turned back within the day. The day after that, all grounddwellers forgot how to reach the cities, what was in them, and even what a seraph looks like. Since then, the seraph cities can still be seen, but not a single person has entered. The extent of the Forgetting is so massive that it's certain they cast a spell - and frighteningly, still keep it up, some two millenia later.

"Nuralk says he met one; he claims they walk like silk in the wind, but can remember little else. One can't help but wonder who else may have met seraphs and forgotten, when even their pictures on Agorisai's palaces were erased."
— Dasnellaar, writer

Theories speculate the Forgetting wasn't intentional, that seraphs have meanwhile died out, or were successfully invaded by the shapeshifter armies after all. It has been in much debate how to handle the situation. All attempts to reach the cities have failed. The Bargassian and Lebridgian engineers can't come up with a solution, and no flyer however good can make the trip. The only sign they might still be there are floatdust miners in Rhagastone, who occassionally are found bound and gagged when they became too clever at the marked borders.

Artists' notes
I used to have them as regular folks around, but then picked them from the list of peoples shrouded in mystery and only to be found later in the plotline. Never fear, honoured audience, all will be revealed in time (I never said when that is, though ;)). Let's just say, seraphs must be one hell of a bunch of speakers, keeping such a spell up for this long.