In the Underdark, there is a species of beings called "Arcadians" which are humanoid in nature. An easy way to think about them is much like the Drow, however the Arcadians can choose to come above ground if they so chose with little to no difficulty. They are concentrated in a singular city called Arcadia. Their city is beautiful and their area is flush with foliage; upon seeing it, you feel as if you are not underground at all but in a vibrant valley that is beautiful, lush, and full of life and light.
Arcadians are an ancient race that do not mingle with other races often if at all. They feel detached from the rest of the races being one of the oldest and their accomplishments in time and space magic make them powerful. Arcadians do not have any desire to be found, and given their exceptionally advanced magic they can move their city/civ around the underground freely without disrupting anyone or anything. Through this, any sightings of them are normally of their scouts and are extremely few and far between throughout history. These encounters are normally an exchange of pleasantries as there are no records of hostile interactions...whether from lack of them happening or lack of witnesses.
Though few encounters are on record, the one commonality between these encounters with the Arcadians is that when one gets close to Arcadia, the stone will sing a light tune. Dwarves, who are renowned for their "intimate" relationship with stone say this is because the magic in the area is so dense, even the earth is happy. In any case however, unless the Arcadians had an objective requiring them to stay, they would be gone from the area before whoever encountered them brought more people. This has also led to some thinking Arcadians are a drunken fool's late night visions that have been passed down via storytelling and the occasional text. However they are very real, and rumor has it if you chance upon them and they take a liking to you, that they may very well help you or bestow fortune to you via their advanced magic.