The problem is, 'nothing' cannot consume 'something'. A building, once created, stands for a long time, and even when it finally crumbles, it is due to wind and rain, more 'somethings'. Darkness realized that it could not stop this threat to it's very being as things stood. Finally, Darkness took action. To destroy the 'somethings', Darkness would be forced to create a 'something' of it's own. Darkness tore it's own being, to do something foreign and create. The 'something' created was named Chaos, and acted as an extension of the Darkness' will.
Chaos set out to undo what Khaine had done. Chaos promptly hit a snag. Created from and by nothing, Chaos simply could not compare to the pure essence of creation that was Khaine. On the plus side, being more than simply nothing, Chaos could think of things the Darkness simply could not comprehend. If Chaos could not utterly destroy Khaine's creations and being through force of will, he would work to subvert and foul everything that Khaine tried to do.
It was at this time that Khaine had taken Maiya as a lover, and Chaos found an opportunity to begin his work. He impersonated Khaine and laid with Maiya, ensuring that the essence of destruction and Darkness would be passed down.
Considering I've heard much about my naming choices, I would also like to point out at this time that all of this is still "beta"... that is, things are still subject to change, including names.