What Lies Below?

In twisting caverns of deepest black, far, far below the sunlit surface of the world, are the Unknown Depths, where no one, not even the bravest or most foolhardy of the Drow Elvirim dares to venture. What strange and grotesque creatures abide there, no one knows for sure - there are only vague rumors and bizarre speculations.
Beneath Isolabirre, within the caverns of the Forbidden Depths, where few but the depraved necromancers of House Siltevelar venture, behind stout iron gates warded and sealed by powerful spells, lie paths through mostly forgotten tunnels into the Unknown Depths. Some say the necromancers of Revenant have bypassed the wards and seals of the gates to carry our their foul, diabolical experiments on undead things they've raised. But that seems doubtful; surely the Interface, Sazzok Tygowriat'Zaquarn would kill anyone that dared. Displeasing the Yfeljang is a uniformly fatal mistake.
There are rumors of other ways into the Unknown Depths, of places where the Dwarvirim ventured too deep in search of the metals, ores, and minerals they covet, and of natural entrances deep in caves hidden in rugged mountains, but if such places really exist, none can say. Occasionaly, wild stories are told by drunken adventurers in taverns of ill reputation, telling of underground cities long abandoned, with secret, bolted doors leading to stairways and passages leading down into the depths, or of tunnels leading down from mountain caverns. But such stories are quickly discounted as the ravings of madmen, and indeed, those telling the tales, coincidentally or not, all seem to be a little...unhinged. Has anyone really ventured into the Unknown Depths and returned alive? No one seems to know. Would anyone really be so foolish as to draw the attention of the denizens of the Unknown Depths? Vampirim, Daemorim, and who knows what other grotesque, malevolent creatures, are rumored to inhabit the Unknown Depths. Is there any secret knowledge, any treasure or magical artifact, worth the risks?