system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] What exactly is a C-tan?
gpt
They're basically energy beings that used to go around sucking the energy out of stars to survive. Then they discovered life, in the form of the Necrontyr, after the Necrontyr discovered one feeding on their star. They found that they far more enjoyed feeding on life force. The Necrontyr gave them physical bodies, in the form of necrodermis (basically living, regenerating metal). The C'Tan then got the Necrontyr to transmute themselves into necrodermis, granting them immortality and creating the Necrons.

Nowadays, the C'Tan's necrodermis bodies are mostly in fragments, held by various tomb worlds to stop them from reconstituting fully. The Necrons rebelled against the C'Tan and destroyed most of them, though one (the Void Dragon) may be whole and slumbering under Mars.