I don't know about "best," but they'd never expect gnomes, pixies, satyrs, wilden or shardminds.
Shardminds. Hmm.
Most shardminds are individual fragments of the Living Gate. These, however, all calved from the same great splinter of the Gate. They fell in a great cluster like a meteor shower and were embedded in a mountainside like a vein of precious minerals.
One was near the surface and managed to free itself. It fell in with adventuring humans, as one does, and slowly began to emulate them, taking on a male gender identity. He was a trickster rogue and got used to hiding his unusual, non-fleshy nature from anyone but his close friends. During their adventurers the group had reason to return to that same valley, in the shadow of the great cliff that sparkled at night in so many colors, and at one point he passed close enough to begin telepathically resonating with one of his fellow shardminds, buried not far from the surface.
He dug it up and began to teach it.
There were no other immortals in the adventuring party, and one by one they retired or died... but by that time the shardminds had begun to find others of their kind, their "kin" if you will, in that mountain. They invested the proceeds from their adventures in a mining operation, and as the area was rich in gold and iron they accidentally became wealthy- and over the generations a town grew up around the mine, and it became a city.
Now, centuries later, the "Foldamers" are a reclusive but powerful clan, the nobility of a city that grew up around them. Nobody knows the truth outside of an incredible loyal cadre of servants and retainers- whose dedication is tested psionically from time to time- and each noble uses magical disguises to "age" and eventually "die," taking on a new identity and continuing the work. Sometimes they pretend to be servants or completely different people, as they don't want their family to be a suspiciously constant size.
They're not particularly afraid for themselves, but they know that many more of their kin are still trapped in the mountain. All the easy, safe rescues were done long ago, and now at best they find another one every thirty years. The problem is that the scattered shards are vulnerable- a landslide might release one, but might also destroy it. Both have happened, but the latter is more likely now. And if their enemies- and you can't rule a city for centuries without making some enemies, to say nothing of aberrational infiltrators and such- found out, it would be the work of days to collapse the tunnels that worm their way through the mountain and kill all their remaining kin.
Of course, when PC adventurers get involved, the secret won't last long at all- especially if they find the one woman who thinks she knows the truth. Her ancestor was the shardminds' first non-adventuring friend, a simple miller who set up shop at the mine and won their trust. After a few generations there was some kind of falling out- not bad blood, as shardminds don't have blood, but some other metaphor (and shardminds are made out of crystallized metaphor, so that's easy!). The shardminds reluctantly erased the memories of the original miller's great-grandson and his family, allowing them to stay in the city and retain the wealth they'd built up associating with its founders... but one of the loyal retainers had written down some things, a list of true and secret names and hints about the origins of the Foldamers.
This list was found many years later and has been passed along by the heads of the noble Miller family as ammunition in a slow rivalry. Every generation or so the rivalry flares into hidden conflict, and what is learned is added to the original document. This "Miller Index" could make the Foldamers vulnerable to
everyone- and could put the PCs in a very dangerous position indeed.
Ironically, the whole problem could probably have been prevented if the Foldamers had learned from different teachers. If the original few had traveled with more forthright folk, they might have just exclaimed, "Woo! We're a big pile of big piles of rocks!" and things could have worked out very differently. Perhaps it's not too late- a safe revelation, a progression toward honesty and true safety, would seem near-impossible... but that makes it a task for heroes.
