This is the peak of the inverted mountain, the lowest area in all Baator, yet it affords the best view of the rest of the plane. The Ninth is a plain crisscrossed by rifts and trenches, cut into the blasted land. Some of these ditches are little more than gashes on the surface of the land, only a few feet deep. Others are deeper than anything a prime could imagine, extending hundreds or even thousands of miles down through the ground. In the deepest pit of all stands Malsheem, the Citadel of Baator. It's a monstrously huge structure that's easily hundreds of miles on every side, including the vertical. It's the largest known Citadel in the Outer Planes, greater even than Khin-Oin in the Gray Waste.
No one's ever gone inside of their own accord, and only the barmiest cutter would even think of it. It's a place of power and pain, death and undeath. Even the gods surely fear to tread there. Of course, it's unmapped and unscribed. The Dark Eight meet in Malsheem, where they are guarded by overzealous cornugons and gelugons, all of whom kill on sight anyone who's not authorized to be here - including other pit fiends, if they haven't been invited.
The River Lethe flows through Nessus, dropping into the trenches and trickling its way across the layer. That's all anyone knows about the layer, for there're few sods who've gotten in - and a berk can count on one hand the ones who've gotten out. The fiends on this level are highly sophisticated, but they don't take chances on deals. This is their stronghold, and they don't let strangers roam around uninvited.
If a body were to get himself invited here, maybe as part of a deal, he could learn all sorts of things. On the other hand, nobody's stupid enough to try to cut a deal with those who control this layer ... are they?