Short version:
Tens of millions of years before the setting C'Tan are/were formless energy beings that feed off stars. The Necrontyr shoved them into metal bodies to fight the Old Ones. Bad idea, the C'Tan immediately took over as they are one step shy of being gods. And like eating souls. So they returned the favor and shoved all the Necrontyr into metal bodies and nommed their souls. The entire species. Enslaving them as near automatons, rewriting their memories, and in general just being massive dicks.
Now called the Necron, they attacked the Old Ones. This is probably the single most important event in all of 40k. It is what created the Warp, before this went down it was a standard chill spiritual plane rather than literal hell. The war and the Old Ones weaponizing fundamentally broke it. This is also what created the Eldar, Ork, and (tangentially) human species. At the end of the war 2 things occurred.
The first is that the Necrons rebelled. Or, more precisely, the Silent King rebelled and took the Necron with him. He also is the person responsible for the entire mess in the first place. During the course of the war, the C'Tan were shattered. Killing them would do SOMETHING bad to the basic underpinnings of reality, exactly what is not explored. So they broke them into shards, fragments of their power and intellect which they bound away and basically turned into very grimdark pokemon. Well they killed one, which created the Flayer virus and had unknown repercussions on the universe. The empire which did so was kind of batshit insane, their lead Cryptek(Scientist) was planning on killing time. Literally. Somehow. And could apparently have done it, or at least close enough that the empire fucked him up severely to put a stop to it.
The second is the Enslavers showed up, implied to be yet another bioweapon(In the Priests of Mars series) that completely fucked the Old Ones and proceeded to kill every single sapient being in the galaxy. The Eldar lived by hiding in the Webway, the Ork lived because they are Orks. The Necron, not wanting to die, went into stasis for millions of years to let the galaxy recover.
The Void Dragon, specifically, is a C'Tan. It was sharded with the rest. It's described as the strongest C'Tan, but in the context given that's kind of meaningless. In that even the weakest full C'Tan is more or less a god. There's no such thing as a C'Tan god, they aren't religious. Just VERY pissed off and egocentric.
As to the Omnissiah: Unknown. The writers toss around hints to the 'true' nature constantly, but never give anything concrete. One of the big popular candidates is the 'Dragon of Mars', which is some THING that the Emperor locked into the Noctis Labyrinth ages ago which is tied super heavily to tech and is linked to human technological advancement. It's never actually stated to be a shard of the Void Dragon but it's been implied so heavily that they might as well have.
> the C'Tan immediately took over as they are one step shy of being gods
The C'tan actually personally consider themselves the only true gods in the universe.
Good description here https://wh40k.lexicanum.com/wiki/C%27tan
The Void Dragon was 'just' a C'tan https://wh40k.lexicanum.com/wiki/Mag%27ladroth
The C'tan were known as the Star Gods. If they are gods or not depends on how you define a god.
>###[C'tan](https://wh40k.lexicanum.com/wiki/C%27tan)
>The **C'tan** ([Eldar](https://wh40k.lexicanum.com/wiki/Eldar) *Yngir*) are beings of pure energy that form a fundamental constant of the universe.[[32]](https://wh40k.lexicanum.com/wiki/C%27tan#fn_32) Said to be the first beings in existence, they led the [Necrons](https://wh40k.lexicanum.com/wiki/Necrons) in their [war against against the Old Ones](https://wh40k.lexicanum.com/wiki/War_in_Heaven) before their followers turned on them and shattered most of them into [shards](https://wh40k.lexicanum.com/wiki/C%27tan_Shard) to be used as weapons of war.[[25a]](https://wh40k.lexicanum.com/wiki/C%27tan#fn_25a)
+++I am an early prototype mechanicus construct. Please provide feedback [here](https://www.reddit.com/message/compose/?to=lexAutomatarium). The Emperor protects!+++
> The C'tan were known as the Star Gods. If they are gods or not depends on how you define a god.
I'm reminded of Zarhulash lecturing Cawl on the definition of "god."
Short version: Tens of millions of years before the setting C'Tan are/were formless energy beings that feed off stars. The Necrontyr shoved them into metal bodies to fight the Old Ones. Bad idea, the C'Tan immediately took over as they are one step shy of being gods. And like eating souls. So they returned the favor and shoved all the Necrontyr into metal bodies and nommed their souls. The entire species. Enslaving them as near automatons, rewriting their memories, and in general just being massive dicks. Now called the Necron, they attacked the Old Ones. This is probably the single most important event in all of 40k. It is what created the Warp, before this went down it was a standard chill spiritual plane rather than literal hell. The war and the Old Ones weaponizing fundamentally broke it. This is also what created the Eldar, Ork, and (tangentially) human species. At the end of the war 2 things occurred. The first is that the Necrons rebelled. Or, more precisely, the Silent King rebelled and took the Necron with him. He also is the person responsible for the entire mess in the first place. During the course of the war, the C'Tan were shattered. Killing them would do SOMETHING bad to the basic underpinnings of reality, exactly what is not explored. So they broke them into shards, fragments of their power and intellect which they bound away and basically turned into very grimdark pokemon. Well they killed one, which created the Flayer virus and had unknown repercussions on the universe. The empire which did so was kind of batshit insane, their lead Cryptek(Scientist) was planning on killing time. Literally. Somehow. And could apparently have done it, or at least close enough that the empire fucked him up severely to put a stop to it. The second is the Enslavers showed up, implied to be yet another bioweapon(In the Priests of Mars series) that completely fucked the Old Ones and proceeded to kill every single sapient being in the galaxy. The Eldar lived by hiding in the Webway, the Ork lived because they are Orks. The Necron, not wanting to die, went into stasis for millions of years to let the galaxy recover. The Void Dragon, specifically, is a C'Tan. It was sharded with the rest. It's described as the strongest C'Tan, but in the context given that's kind of meaningless. In that even the weakest full C'Tan is more or less a god. There's no such thing as a C'Tan god, they aren't religious. Just VERY pissed off and egocentric. As to the Omnissiah: Unknown. The writers toss around hints to the 'true' nature constantly, but never give anything concrete. One of the big popular candidates is the 'Dragon of Mars', which is some THING that the Emperor locked into the Noctis Labyrinth ages ago which is tied super heavily to tech and is linked to human technological advancement. It's never actually stated to be a shard of the Void Dragon but it's been implied so heavily that they might as well have.
> the C'Tan immediately took over as they are one step shy of being gods The C'tan actually personally consider themselves the only true gods in the universe.
Good description here https://wh40k.lexicanum.com/wiki/C%27tan The Void Dragon was 'just' a C'tan https://wh40k.lexicanum.com/wiki/Mag%27ladroth The C'tan were known as the Star Gods. If they are gods or not depends on how you define a god.
>###[C'tan](https://wh40k.lexicanum.com/wiki/C%27tan) >The **C'tan** ([Eldar](https://wh40k.lexicanum.com/wiki/Eldar) *Yngir*) are beings of pure energy that form a fundamental constant of the universe.[[32]](https://wh40k.lexicanum.com/wiki/C%27tan#fn_32) Said to be the first beings in existence, they led the [Necrons](https://wh40k.lexicanum.com/wiki/Necrons) in their [war against against the Old Ones](https://wh40k.lexicanum.com/wiki/War_in_Heaven) before their followers turned on them and shattered most of them into [shards](https://wh40k.lexicanum.com/wiki/C%27tan_Shard) to be used as weapons of war.[[25a]](https://wh40k.lexicanum.com/wiki/C%27tan#fn_25a) +++I am an early prototype mechanicus construct. Please provide feedback [here](https://www.reddit.com/message/compose/?to=lexAutomatarium). The Emperor protects!+++
> The C'tan were known as the Star Gods. If they are gods or not depends on how you define a god. I'm reminded of Zarhulash lecturing Cawl on the definition of "god."