The Dark King

The Dark King is / was a potential Chaos God that was almost incarnated during the latter days of the Siege of Terra.

In much the same way Slaanesh was born from the civilisational death of the ancient Aeldari, the Dark King would have been created from the same symbolic death of Humanity during the Heresy.

Whilst Horus was presumed to be the being who would become the Dark King, the daemon Samus also hypothesized that both the Emperor and even Erebus were potentially in the running for it, which is interesting for the former and baffling for the latter. The treacherous dickblister isn't in the same league as the other two.

Obviously, it didn't end up happening.