Location: parallel demiplane, in the foothills of the southern Tharis mountains
Race: drow (major), wood elf (minor)
Rulership: council of elders
Rise: between -10,500SG & -10,000SG
Their only major ally is with the wood elves of Tharis forest, and even this is a secret to your 'common folk' among them. Only the highest council members of the wood elves are aware of their existence, maintaining contact and assistance through wood-elf intermediataries.
Since the descent, Eilistraee has gained primary worship amongst the primarily-drow group. Since the Godswar, she has since been viewed in joint aspect with Sehanine, a stance that is gladly accepted by the mixed elf/drow group. Similarly, the various deities of the Seldarine are worshipped here as they were of old.
This mixed drow and elf faction consists of a group of the direct descendants of the Vandor'ivae elves. Unlike nearly all primarily-drow groups realm-wide, they are not evil, nor 'drow' in nature; their history, rituals, language, and nearly all else is elven, and is directly taken from their heritage as original inhabitants of Daoine'yndaar.
Historically, in the years following the Mormhaor’Sykerylor (killing storm) that destroyed Daoine'yndaar, the wood and dark elf remnants of that city banded together to steal their city away from mortal hands, that it may never be sought or abused again by outsiders. The ruins of the city, still alive with the warped energies of the storm and the now-twisted mythal, were shifted to a parallel demiplane (thus why there's no ruins of the city left in Tharis forest). A large number of the elves gave up their lives to enact such a powerful ritual, and of those who were left, a group were tasked with defending the hidden ways evermore, should the day come when they might be restored. Of course, whether that is even remotely possible, given the corruption of the ruins, is probably unlikely…
Their descendents live to this day within the southern limits of the parallel demiplane, crossing to the prime as necessary to maintain contact with their few allies, and to trade. A series of way-gates provides a pathway via the ethereal plane, able to be opened only by those trained to attune each gate. Of the descendents, they have continued much into the habits of their ancestors, with the wood elves following primarily martial pursuits, walking their borders and serving as the guardians of their home. The drow continue to maintain their arcane inclinations of old, maintaining the lore of their people, upkeeping their wards and other magical defenses, trying to keep the corruption of the ruins in check, and guiding those who walk the paths between their realm and the prime. Divine callings are strong within both groups, with priests in service to Eilistraee and the Seldarine. Druids exist, but are rare, given the recent return of their gifts, and most are still trying to learn from the records of the old ways, or from envoys from Tharis forest.
Of note: 'Tyrnea'anea: the Children of the Night'. A term used to refer to the 'drow' among them, as their wood elven brethren do not consider them to be 'drow' as other elves use the word.
Once the arrangements are made, Ilmffyn leads you both into the mountains, south and east of Tharis. At several points, you realise she is somehow opening or attuning gateways along your path, and each time after a pause, you step through stone pillars, or a tunnel, or what appears to be a naturally formed earth circle, to find yourselves somewhere completely different. Beyond the first, you are met by a wood elf whom she introduces as Koehdeth, and reintroduces herself as Il'myfir - her true name, elven rather than drow.
The pair travel onwards leading you. Koehdeth seems to be her escort and protector, though you wonder that there is something more between them than mere friendship. Passing another of the waygates, or way-points, or whatever they are, and with both of you having wielded powerful magical arts at some point, you would recognise the muted and fuzzy feeling of the ethereal plane as you pass walk down through a chasm within the mountains, and when you reach its end and pass through a strange curtain of shimmering air to step out on the other side, you are somewhere… else.
A forested glade fills a vast valley within the mountains, leading back down towards the north and the Tharis forest you're familiar with. But all you can spy in that direction, as far as your vision stretches, is a vast sea of trees. You're sure the region is not so richly forested as it appears… but then, nor was the mountainside leading up here. It's akin to the world you know, and yet not. And above the forest itself, far to the north, lies an unsettling gloom that shifts and flares and dies back again, sparking with nameless colors and auras, like nothing you've seen before.
Il'myfir leads you to her home, at the southern edge of the forested valley, nestled up against the mountainside. It is here that you finally meet her people, whom she calls the Oussea'ghymn - an elven word you would understand as 'Heirs to the Forgotten Ways'. You are struck first by the fact that while most eye you with curiosity, not a single hostile look or word greets you, nor any attempt at false sycophancy. The feeling is incredibly odd, after having spent so many decades accustomed to as much from both elves and drow alike. They are inquisitive, but polite, and otherwise entirely accepting. You are secondly struck by the fact that the people of this place are both drow (known here as Tyrnea'anea, 'the Children of the Night', rather than dark elves or dhaerow) and wood elves, living in apparent harmony with each other, as though they had never known anything else.
The Oussea'ghymn are led by an elected council of elders, in a similar but more organised fashion to that which you'd be accustomed to amongst the wood elves, which you'd be able to find out from their records, was adopted following the Crown Wars and what they considered 'maligned leadership' among the sun elves - that bloodlines were the rule of law. Instead, their leaders are elected by the vote of all, and favored for their wisdom and leadership. They would meet shortly after your arrival, and offer you a formal introduction as well as the hope that both groups may take up friendly terms, if so desired. Though they would again express their wish that the nature of their home and their ways does not pass the two of you (and possibly a few others of the highest among your houses, if they are beyond doubt of loyalty).
Their home is a fascinating amalgam of styles and substances, yet all somehow bound together in a harmonious fashion. Dwellings exist both above and beneath the ground, and are made of both wood and stone. You get the impression that they were formed or grown that way (likely with the help of magic), with stone that seems risen from the mountains itself, jutting to shape columns and spirals, and form foundations for the trees and vines that snake upwards to flare their limbs as balconies, or ensnare the roof of their cavern to form supports themselves. While it seems that most of the drow live beneath while the wood elves live above, there doesn't seem to be a true segregation of the two groups; it seems chosen simply of comfort and convenience, and the groups intermingle freely. Family units are mixed groups, sometimes with partners or children of differing heritage. You soon realise, though, that it isn't even a deliberate choice as much as the result of prior millennia of even a few mixed relationships, and the pure-breeding nature of elves; some pairs of matched mates still occasionally walk with children of completely different appearance.
A comfortable home is offered to you for your stay, one that touches both above and below as it finds its way through tunnels in stone to open upon sky above. You have enough time to mingle and learn a little of these people, who carry very little of the distrust usually held by elves and drow that you meet. They speak quite freely of their ways, and answer most questions directly.
It would be also warned not to travel beyond the borders of the mountainside further north, where the guardians keep watch against the twisted energies and denizens of what they explain is the ruins of Daoine'yndaar. Very few among their number pass these limits - only those most potent in the magical arts, who are tasked with the revered duty of ongoing stewardship of the ruins. It doesn't take much to gather that these people consider themselves the guardians of their ancestors' heritage - guarding against any coming in to steal it, or anything malignant escaping what is left of it.