The C'tan originally lived as energy beings that fed on the stars themselves. They are also called star gods or star vampires. They were transferred to the physical world thanks to the technology of the Necrontyr.
The C'tan used the hatred of the Necrontyr towards the Old Ones to help them gather the more subtle living energy they now craved. The C'tan hate the warp and have thus set up a series of 'pylons' on Cadia, which, when completed, will close off the Warp from the material universe entirely, utterly destroying any living creatures with a soul.
In the war between the C'tan and their new slaves the Necrons against the Old Ones and the younger races, their red harvests started to run dry. Then the Eldar Laughing God tricked the C'tan into consuming each other until only a few remained. Not long after that a plague of Enslavers emerged, which apart from destroying the Old Ones, also depleted the red harvests further. Thus the C'tan decided to enter stasis until life repopulated the galaxy and their harvests could begin again.
The four known remaining C'tan are:
- The Nightbringer impressed its image as that of the grim reaper itself on the psyche of the younger races, apart from the Orks (since they do not fear death). Upon entering stasis it was almost destroyed and starved but was released accidentally by Space Marines.
- The Deceiver came out of stasis an unknown time ago and has been weaving plots ever since, including the destruction of the Eldar weapons Talismans of Vaul designed to destroy the C'tan on their emergence.
- The Void Dragon the most powerful C'tan, resides in stasis, theorized to be located on Mars and is rumoured to be the Machine God venerated by the Machine Cult.
- The Outsider became insane due to its consumption of other C'tan, a trick played on it by the Eldar Laughing God. It is currently imprisoned in a Dyson Sphere under the Galactic plane. The Eldar Harlequins (Followers of the Laughing God) Whisper that " One dark night, it shall return".
The Inquisition employ elite Temple Assasins, one of which, the ones from the Callidus Temple use a weapon called a C'Tan phase sword. It is unknown what specific relation the weapon has with the C'Tan themselves. However, the most likely explanation would be that the C'Tan phase sword is made out of necrodermis, as explained in the Necron codex, when a Callidus assasin attacks a governer, only to have her C'tan phase weapon absorbed into the "governor", who is most likely the Deceiver.