The Demon Dimension / Hylden Realm
The Hylden Realm is a demonic dimension that warps and distorts reality with chaotic energy. The Hyden tried and succeeded in carving out a niche that left a few of them unaltered. Using the Glyph network, they created order out of Chaos, even subjugating the native creatures there.. but it was very limited and at great cost. Eventually even their great magics would succumb to the demon dimension and warp. The Nexus stone and device were all part of their attempts to come over. The Demon Realm is not a mirror copy of the Material though, it just touches on it in certain 'weak points' like Avernus. It's not like the Spectral Realm.. it has its own areas, too and time passes differently there as well.
In [The Dark Prophecy] I was able to add to the Hylden mythos. Imagine a race that was able to fight the ancient vampires toe to toe to a standstill. They've been warped, weakened, and drained through their experience in the Demon Realm, but those that preserved their pure state in the center of their Glyph cities are every bit as strong as before.
Like any intelligent species, the Hylden have their own hierarchy, conflicts and insurrections. The demon realm would have necessitated a certain unity though, for the sake of survival. I do like to think that there would be have and have-nots, with lesser clans left to the warping as their abilities faded first. Remember that nothing and no-one in Nosgoth is one dimensional. If we had the time, we would have woven a lot of dimensions into everything.
The Hylden appear in the demon dimension, obviously intruders to that realm, which causes immediate conflict. For the Hylden it would have been like being damned to hell: they fought and carved out their place in the world. Preserving what civilization and technology they could. The more damning realization came later when they realized the entire dimension was warping and twisting them. They used their sorcery and Rune technology to create bubbles that kept them safe until those too started to twist and fail.
Further still they realized that they could not die from old age, essentially freezing them in time and space (I think unable to raise children as well. Realizing their eventual fall was due, they reached out to their old enemies and forged alliances, finding ways to breach the thinner passages to the Material Realm of Nosgoth. (Entropy has a way of making strange bedfellows) Lesser demons would have been enslaved, greater demons would have been in an uneasy alliance with the Hylden. They Hylden would have used their possession abilities to reach through the jail of the pillars and even possess demons who came through the other side (thus their knowledge of Kain and Raziel). In Defiance we decided to use the 'green glowing eyes' to show possession or influence. I also believe they focused on possessing more pliable vessels. (Also, I think the demons were pissed off about the whole possession thing)
This vast glyph powered city, with its edges being warped by the demon dimension. The glyphs would keep the warping at bay, but they would eventually fail and a whole section would fall to the demon dimension's twisting. It would set up tension within the city, intense resentment, and conflict for resources. I'd really like to add dimension to the Hylden.