The Abyss of Doom is a natural landmark of the subterranean world of Kazrrad, and it lies beneath the Nohalion Mountains. The deep formation was a major barrier in ancient times, but has long been conquered by engineering. It is a feature to the city of Negeémiliel. Once being a 'bottomless' moat, now being a void near the heart of the city, nearly besecting the city, in physical and cultural terms.
The "Abyss of Doom" is indeed an abyss, an immense crack that may be over a hundred kilometers long, that almost cuts the underworld of Kazrrad in two. The abyss is also extremely deep, its depth being comparable to its length, being a dozen kilometers deep from its top at least. The crack is relatively quite narrow, even so, any tunnel that is cut through a wall of the crack would be confronted by an unimaginable void. The tunnel would be confronting a wall of nothing but air, going seemingly infinitely side to side and down, and in many cases even up. The rock wall on the other side being possibly half a kilometer away.
The Abyss forms a great void within a voluminous cavern that is the setting for the main and central part of the vast city of Negeémiliel. The abyss is a tremendous crack across the floor and the lower part of the east and west walls of the cavernous gallery. A crack that extends essentially straight down, for 12 kilometers, perhaps over 10 kilometers below sea level?
How wide it is hasn't been recorded, but the building of the original bridge is described as a major engineering feat, requiring many lives of slaves to construct, suggesting a fairly impressive span. Additionally, there is room and need for many other secondary bridges to be made, so that suggests that there is a broad gulf indeed? The bridge is stone, and not a suspension bridge, so the span cannot be too wide either, perhaps 50 - 100 meters across? It is true that the cavern of Negeémiliel is the highest roof of the abyss, some say the apex of the vault of 'True Kazrrad'. As the crack does indeed narrow and reaches its end in this cavern, far above sea level, there is some truth to the statement.
More than one bridge connects the two sides of Negeémiliel, but the bridge going to the Temple of Doom is the main bridge of stone footing that can carry enormous weight and traffic as throngs of drow, food and merchandise make their way to the temple and an open market that operates within and in front of the imposing and rambling temple on the south side of the abyss. The secondary bridges might be of a suspension type made of massive silk cables that allow ample foot traffic, but don't lend themselves for carts going in two directions, quite likely the traffic on these secondary bridges is one way only.
One might speculate that the side cross section of the abyss and the cavern that Negeémiliel fills, is akin to the head of a flower atop a long stem? One might also speculate that there might be caverns similar to the size as Negeémiliel's at different strata on the way down? Also, these caverns and this shaft are part of the subterranean world of Kazrrad, and so, may be artificial in their creation as opposed to being formed purely through natural forces? These specifics are unknown.
It is known that the bottom of the shaft is inhabitable, as a drow expedition from the Negeémiliel University has been down there. A tribe of extremely primitive demons exists down there, and kobolds. There are uncategorized other deep monsters down there as well. This demonstrates that this is another major 'deck' to the artificial cavern system that is kazrrad.
The shaft is voluminous enough, with sufficient breathable air, to make the great city of Negeémiliel possible. One might speculate that the shaft was deliberately placed by the primordial 'world builders' as a means of ensuring a connection of atmosphere from the upper regions of kazrrad, down to its 'basement' level? Or as an 'elevator' for moving things from top to bottom, or bottom to top of kazrrad?
Negeémiliel has come to be built on both the south and the north sides of the abyss, structures built on the floor of the cavern for kilometers in both directions from the abyss. the large number of bridges having made the trip across the abyss an easy matter. On the south side is built the citadel of the "Temple of Doom, named for the abyss. The temple having evolved from being a fortified trading post on the periphery of the drow city-state of Erehel-Sinu to being the heart of a great capital.
The Abyss of Doom was an obstacle to the southern reaches of kazrrad in ancient times, and it became desirable to span it by the drow civilization of Erehel-Sinu. This bridge which was at least started at the year 2440 b.a.H., and upon the completion of the bridge, on the coveted south side of the expanse, the original footprint of the Temple of Doom was the original citadel, settlement, fortress, and temple in one, called "Renden d'Haruk" that served as a fortress and trading post for the trade coming across the bridge from the south of Kazrrad, the bridge being one of only a few ways across, at that time. Also the only one for hundreds of kilometers. Later, as the city swelled to fill the vast cavern around the Abyss of Doom, the citadel became increasingly devoted to religion alone, and became the main temple of the city, although it always had many different functions, being a sprawling, multi-structure facility.
A temple seems to have been placed on the site from early on, probably devoted to the goddess Lolh by the Erehel drow. Many slaves were lost in the construction of the bridge, it seems that for quite some time, sacrifices were carried out by the casting of selected slaves down into the abyss. Quite possibly this started as a commemorative event of the construction of the bridge, and of the growing city. These sacrifices were ended at some point in the past, quite possibly when the city achieved independence and the formerly slave and lower caste Negeél people took control in 480 a.a.H., or also quite likely during earlier reforms as the Erehel elites made many concessions to the Negeél ethnic majority in the city?
It is also probable that the bridge(s) were destroyed or damaged several times in its history, especially during the period of 50 - 100 a.a.H., as during the second half of the first century a.a.H., great geological upheavals took place, and great sections of the kazrrad caverns were destroyed.
Negeémiliel has become a great metropolis, with many established trade routes and tunnels to the surface, with trade very important. Huge amounts of trade still come over the bridge over the abyss, but what was once a great, imposing façade for the city's main gate, entering the Temple of Doom, has become instead built over, and the 'backdoor' to the metropolis. There are now 80 gates to the city, with the Temple of Doom and the main bridge over the Abyss of Doom only one of that number. Indeed, the abyss and bridge are well in the heart of the city, as Negeémiliel now extends far around to both the north and south of the abyss and bridge, in the southern part of the kilometers long gallery, and into the caverns south of the gallery.