It's fitting that the city of the dead is known as the Maw, as it is the literal mouth of the dimension leech known as (by its now dead victims) the Inevitable, for being consumed by it is as inevitable as death. The only creature that knows more about it is the All Consumer, and it knows of nothing of activity outside of Solinia's planet, due to it being so deep underground. All that's known is that the Inevitable is like the All Consumer in that it wants to eat everything, except it doesn't just eat physical things--it latches its mouth and sucks a whole dimension into itself. And unlike the All Consumer, it is in no way crippled or hindered in its progress. Starting as a giant leech no larger than a moon, it sucked up its first dimension with relative ease. It moved onto countless others. Presently, there are only a few dimensions it has not eaten. One is the Dream World. The other is Solinia. Not knowing that dead souls are drawn to the Maw with the intent of consumption, the only thing that keeps Solinia's dead from its belly is the fact that so many souls and debris are packed into the Inevitable that it can no longer swallow. This has resulted in the dead building the city of the Maw on its victims, right above its throat. This also resulted in the Klacker's introduction to Solinia, as it was one of the few survivors of its last meal. The Klacker, which avoided being swallowed, fell out of its mouth and right onto Waeto. It is possible that there may be more creatures from the Klacker's dimension hiding in its mouth. The Inevitable has also eaten portals, preserving them in its mouth and allowing escape.
The time draws nearer when the Inevitable will be able to eat Solinia, however, as the Inevitable is a slow digestor. Once it has enough room, the bodies will shift and the Maw will sink into its throat. That is when it will have enough room to fit in just one more planet.
The Inevitable is a leech almost as large as an entire dimension. This is because its body is elastic enough to fit many universes, but tight enough to squish them all down. If the Inevitable were to be emptied, it would look like a loose banner flying from Solinia's planet.
As a defense mechanism, the Inevitable can blend in with the environment to escape detection. This doesn't work on its insides, however, giving the Maw the appearance of a red hole in the sky.
Note: The mouth should not only fit onto Solinia so it can suck the life out of it, but it should look like a giant portal to the Maw hanging in the middle of the sky, as only the Inevitable's insides can be seen when it's invisible. Another way to put this is to think of its mouth as a substitute for a moon or star.