C'tan
The C'tan are ancient and powerful Star Gods given physical bodies by the ancient Necrontyr. They dislike anything Warp-related, and may have tampered with humanity to inject the Pariah gene. Can not directly enter it either. In 4th edition they were responsible for massive amounts of just as planned.
The first encountered was The Nightbringer, who was found "sucking" the energies of the Necrontyr planet's sun, the same sun that gives off enough hard radiation to make proto-Necrontyr life short, nasty, short brutish, and short.
The Necrontyr formed a necrodermis body and coaxed the star-sucker to inhabit it. This was C'tan later named The Nightbringer. When the Nightbringer awoke in the new body, it noticed the Necrontyr for the first time and discovered they were like Chinese take-out: really yummy and spicy, moreso than bland stars, but you could eat a few thousand and still feel hungry an hour later. It took a while and much slaughter before the Necrontyr could convince the Nightbringer that they were more useful in servitude, and there were other yummy lifeforces to be eaten out there.
Other C'tan gods were discovered feeding on stars, but the most significant of the early ones discovered was named The Deceiver. Weaker than the others, The Deceiver is better at leveraging other entities to do what it wants. After inhabiting a necrodermis body, it adapted to the material world quickly and became very popular with the Necrontyr by means of cunning and guile. It was The Deceiver that convinced the Necrontyr they could escape their short lifespans and succeed in their battles against Old Ones by inhabiting necrodermis bodies themselves. The Deceiver neglected to mention that these new Necrons would be insensate, dull-witted and easily manipulated by the C'tan.
During the Necrons's war against the Old Ones, the C'tan also fought amongst themselves, destroying the necrodermis bodies they used to have an effect on the material world. By the time the Old Ones were rendered extinct, only four C'tan still existed in the material world: The Nightbringer, The Deceiver, The Dragon and The Outsider.
Known C'tan
The Nightbringer - Here to fuck your shit up. The first C'tan to inhabit realspace. Scared the crap out of a race/culture that was a threat to the race/culture that created all the other races in WH40K. Likes killing, killing and, well, killing. In short: batshit insane. Ate most of the other C'tan.
Tyranid edit: Have fun being instant deathed. :D
The Deceiver - Dickery on a galactic scale, it even out-dicks Eldrad. Got the bright idea of convincing the Necrontyr to turn themselves into the zombie-robots known as Necrons. Rumored to have implanted the Pariah Gene into humans to help combat the forces of the Warp.
The Void Dragon - Can exert control over any machine. Able to communicate with little children, regarded as the one C'tan with the most power over the physical world when at full power. Rumored to be taking a nap inside Mars and to be worshipped by the Adeptus Mechanicus as the Omnissiah. Also rumoured to have visited Earth during the first century A.D., where the Emprah defeated it and sealed it to sleep under the surface of the "wandering star" (Mars). There's also an Eldar legend of their forge god Vaul failing to destroy The Dragon but managing to get it to hibernate in a place called "the Vaul Moon."
The Outsider - Ate at the same C'tan-cannibalism saladbar as The Nightbringer, but feels bad about it; blames Eldar gods for his bad eating habits. Decided to say "screw you guys, I'm taking my ball and leaving," and currently lives in a dyson sphere outside the galaxy. A hive fleet of tyranids decided to go AROUND this place, not just ignore it like tyranids do with Necron tomb-worlds. Were the tyranids scared? Dunno, it's safer to lick a bandsaw than stop to ask a tyranid.
Gallery
If someone actually fields these miniatures on the table, you're getting screwed.
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The Nightbringer
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The Deceiver