Aza'Gorod, the Nightbringer is one of the C'tan.
Official Canon Edit
The Nightbringer is one of the most powerful and terrifying C'Tan who participated in the War in Heaven. He dueled against the Eldar god Khaine, and though he was defeated, he left a lasting blow that created the fear of death throughout all soul-bearing creatures. He was convinced by Cegorach and the Deciever to start eating the other C'Tan. Like the other surviving C'Tan, he was betrayed by his Necron subjects and broken into shards. One of those shards was unwittingly freed by Uriel Ventris.
TTS Canon Edit
Decius mocks the Nightbringer by claiming he has no friends.