The Dark King is a theoretical fifth Chaos God that has been prophesied by both Eldar and Humans throughout history, but is first mentioned overtly during the final stages of the Siege of Terra in the Horus Heresy. The Dark King represents the drive to complete destruction without reason or aim.
While initially speculated to be the Chaos-corrupted Horus or the Daemon Samus, it eventually becomes apparent that the Dark King was destined to in fact, be the Emperor of Mankind.
