Choronzon

Choronzon is oine of the least understod entities known, and even then, known only by a few. Named by occult scholars as the Dweller (or Watcher) of the Abyss, to encounter it is to completely obliterate the mind of most sane beings. Named as such it seems to dwell in a region known as the Abyss, and not the blasted hellscapes ruled over by the fiends that call their home as such.
This Abyss seems to be completely separate, far more ancient and otherworldly. Descibed by many who study such Planes, it is a place described as being one of three primordial realms along with the Void & the Empyrium, and utterly desolate. Those who have viewed it describe it as dreary at best, and literally soul destroying at worse sucking the life & colour out of everything it touches. Made up of cosmic detrius & darkness, along with the unmistakable touch of chaos, this realm exists between the Mortal Planes & the Heavens. Some have mentioned that it helps to separate the two, but again, that is conjecture.
Choronzon lies within this domain, able to percieve everything within it at all times. It does not matter how far the distance or what it is, he knows constantly watching.

Summary

Whether called the Dweller in the Abyss or the Watcher in the Abyss, the only sure thing that occultists can say is that Choronzon is ancient. To say so would be an understatement. As a being of discarded elemental matter with a trace of all magics (as well as the Heavens & Material Plane) that have existed, exists, and will exist, he is fundamental to the Abyss itself. He is at both times a castoff & a balancing agent, at all times guarding the Abyss & making sure it is not took over by any outside power.
Created at a time when the first physical Planes & Heavens arose, the Abyss served as a means to separate them from one another & a dumping ground for cursed or unwanted ideas, dreams, souls, and even worlds & Planes. Still used in this manner, it has always been used for the purpose to stop both intermingling & corruption by one another. It actys both as a barrier and a place for the 'cast-offs' so to speak are dumped, for wont of a better term. This in turn, led to it needing a permanent guardion & overseer, something that could withstand the effects of the Abyss itself, and the two opposing Planes. The earliy deities & entities randomly chose a creature of the Far Realms to fill this role knowing that such a creature could easily deal with the pressures of such, and not care one iota about the squabblings of gods & mortals. Unfortunately they chose Choronzon.
Choronzon has been proven highly effective in this regard, keeping the Abyss in some semblence of order so to speak. Unfortunately, he is completely alien in mind & body to all others who have a stake in keepng the Abyss as a dividing line. It appears that he is in some way related to the Star Gods, but doesn't care much about them or any deity for that matter, looking upon others as a plaything at best (out of boredom some would say), or asa souce of nourishment.
As the Abyss is used as a cosmic dumping ground for those not worthy for a mortal life / physical form or into either the Heavens (and even Hells) that exist, Choronzon has built up a vast array of potential souls, items, and even worlds. The cast-offs exist in some form in the Abyss being fed on by him as nourishment, and even the odd entity & god has suffered this fate. Choronzon cares naught from where these come or what their pasts were.
This has also lked to him being linked directly to the Abyss now as both him & it affect the other directly. He views it as his personal fiefdom, and takes it so seriously that even the most powerful of entities are likely to be harmed or killed on sight by him if they make a move in any way against the Abyss.
It surprisingly has a couple of benefits to both the Afterlife & the Material Planes. These are the following:

• Nothing that ends up here is truly forgotten or lost forever. It is highly likely Choronzon himself will remember it, and in almost perfect detail.
• Choronzon, and thus the Abyss, acts as a perfect balance between the afterlife & the Material Plane. Nothing can get through from one to the other without him knowing about it, and stopping all but the most powerful getting through.

A few occultists have actually stated that they have peered into the Abyss and were driven insane by what stared back..... Choronzon himself. He is well aware when any do try to scry into his domain, and will regularly appear in some way to prevent them from doing so. If, and only if, the scryer manages to placate him with something applicably worthy (usually never heard before knowledge or something else of legendarily rare quality), then he may barter for it. This can range from the soul of a powerful rare entity to the meaning of life itself, or even the means of immortality, it can be bartered. Such things are given freely to Choronzon, for what he can give in return can be equally powerful.
If tricked though, it is likely that Choronzon will lash out in some unforseen way, and the entity making the bargain may find that they have a one way trip to the Abyss.

Spread

Known only to a few in both occult and religious circles, Choronzon is rarely encountered outside the most dedicated of these scholars.

"You do not want to look into the Abyss, you do not want to see what will be staring back at you. A creature of absolute nightmare. A fallen one of extreme power & influence is hidden there. A guardian and a dark reflection of those that bring us the light of the heavens."
The Seeress, in the Tower of Emon Kanis, Aachen.