I know Zon-Kuthon has no affiliation with the Dominion as far as his origins are concerned but what about Zon-Kuthon and the Kyton (sorry to be tangential)?
There appears to be a clear connection between the two, though never stated.
In my campaign, theologians debate these topics. Did Zon-Kuthon create the Kyton in his imagine? Were the Kyton divine beings loyal to Dou-Bral prior to his transformation and were corrupted by his prescience? Or did Dou-Bral find them at the edge of existence and was corrupted by their dark workings?
Personally, I've always thought Zon-Kuthon was the Kyton's greatest convert. He is beholden to them, yet they are not his master. Rather they see themselves as equals, linked by their common philosophy of pain, sorrow, and bleak enlightenment.
As to Zon-Kuthon's connection to the Dominion, perhaps their is one through the Kyton?
Let us say the assume that the Dark Tapestry is just what it claims to be. The nothingness between stars. Let us also assume that the edge of the universe functions the same in both worlds, and the further to the edge you travel, the great the distance between stars. Eventually, there is just nothingness. A void of light, sound, sensation, and matter. The Dark Tapestry.
Now imagine the hopelessness one would feel staring into that endless all-consuming nothingness. The nothingness, you would eventually imagine, is so much greater than the substance of the universe. The universe is finite, surrounded by an infinite absence. Thus, the universe must be insignificant in comparison to the void. A figment or light in a reality or darkness. It would eventually break anyone's spirit.
From Serenity: "They got out to the edge of the galaxy, to that place of nothin', and that's what they became."
Now, how did nothing turn Dou-Bral into Zon-Kuthon? Each deity represents an ideal, which they hold higher than themselves. Asmodeus esteems tyranny, Dou-Bral esteems beauty, so on. Just as mortals search for proofs to justify their faith, so to did the gods search the cosmos for evidence of these ideals. Dou-Bral sent his divine servants out to the farthest reaches of the universe to find true beauty. Eons later, the Kyton returned. They claimed that true beauty was born in pain. That one would understand beauty only in the moment that they experience true pain. Beauty was a selfish expression of meaning to temporarily dull the pointlessness of existence. They claimed they had gleamed this knowledge from the gods in the blackness between stars. Uncaring gods. Massive eternal gods. All-powerful gods. Gods of nothing. Horrified, enraged, and curious, Dou-Bral traveled himself into the Dark Tapestry, looking for these uncaring gods who twisted his servants. Countless centuries passed as Dou-Bral trekked deeper into oblivion. Finally, he realized the Kyton had simply gone mad. There was nothing out there. No truths. No beauty. No gods. Just oblivion. A vacuum absorbing all sensation and crushing it into nothing. Dou-Bral turned back, only to realize he had lost himself in the nothingness. He was alone. Frustration turned to fear. The nothingness was so massive, how could he hope to find the universe within it? He was lost and made utterly insignificant by the massiveness of the void. Then, it clicked. He was no different than the universe itself. It, like him, was a mere droplet in the ocean of black. Yet, why? Here was the truth. And what a beautiful truth it was, though he was not yet ready to understand. There were no gods in the Dark Tapestry, the Dark Tapestry was the god the Kyton rambled on about. Not a god though, something greater. An ideal. No, more than that. It was the reality behind the ideals which he has searched for. Life was alien to the void, and the void foreign to life. The two were in conflict. Though existence could be made meaningless by the void, experience allowed life to defy annihilation. Only vivid sensation could alleviate the numb oblivion. "Existence is such only to challenge the nothingness." This was the first truth. Isolated from everything, Dou-Bral tried to generate experiences. His mind had long forgotten the colors, smells, tastes, or sounds of others. But even in the grips of nothingness, he had himself. He talked and sang to himself. He smelled his flesh. He stared into his hands. But eventually, those sensations dulled and lost meaning. He screams. Gnash his teeth. Bit at himself. Clawed his flesh. Drew patterns with his blood across his lips. Each time the void returned, Dou-Bral ravaged himself further, enjoying the brief respite from nothingness a little more than the time before. "We are alone in oblivion, and we endure only through selfish experience." This was the second truth. Tearing at himself, Dou-Bral turned forever inward to endured the bleakness. He focused on the anger, the sorrow, and the grief of his existence. His physical, emotional, and spiritual pain gave him strength. He would not succumb to the void. Pain overcame the void. The void had no feeling. It could never understand pain. It was powerless against his pain. This was the final truth. "Pain was the most powerful force of all. Pain is a blessing. Through pain, we endure."
It was by chance that Dou-Bral noticed the faith glimmer of a star and escaped the endless prison of the Dark Tapestry. Yet, to him, it was sign of his victory. The void could not consume him. He had won. He had been rewarded. He had come to understand truth and now he was ready to share it with the world. The void must be challenged. Nihilism must be crushed. Existence must endure. Life is pain. Thus, Dou-Bral was gone, and Zon-Kuthon was born.
The Kyton, without Dou-Bral guidance, abandoned their station and former master, choosing to migrate along the lower planes refining and teaching their twisted philosophy. Zon-Kuthon cares little about their independence, for what use are servants when we are all alone? He needs nor craves no servants, only fellow comrades in truth. In this way, Zon-Kuthon views the Kyton as allies, though loss allies at best. And those Kyton who encounter their former master simply smile, proud in the knowledge that their guidance has converted a god to the true path.
Then what is the Dominion of the Black and who are the gods the follow?
As foolish mortals seek the truth in the darkness between stars just as the Kyton and Dou-Bral had before, they too fall victim to its crushing madness. They give the void divine purpose and prophetic goals. They give it traits and characteristics befitting its assumed power. They call it Yog-Sothoth, Nyarlathotep, and Azathoth. They describe it in abstracts as colours out of space, swirling bubbling masses, writhing black tentacles, eldritch aeons, or cyclopean geometry. But it is merely their tiny mind's trying to grapple with infinite nothing. They are broken body, mind, and soul by it. They are one of the Dominion of the Black. And their psychotic devotion to these non-existent gods is so strong it can generate divine magic, tapping into and simultaneously feeding the same well of faith that the Kyton and Zon-Kuthon unwittingly share.
Friedrich Nietzsche "Battle not with monsters, lest ye become a monster, and if you gaze into the abyss, the abyss gazes also into you."
There is nothing out there at the edge of existence. No power. No truth. No gods. No meaning. Just nothingness between stars. But that nothingness is so vast, so all-consuming, that those who confront it must contend it to be the greatest force encountered. It is only what we give it. Some call it The Uncaring God. Some call it the Philosophy of Pain. Some call it The Truth of All Things. All know it as The Dark Tapestry.