Night Lords and the Dark Gods...

[Dammeron]

The Night Lords are an example of a completely secular Chaos Legion; they regard the religious and superstitious pursuits of their fellows with utter disdain. However, this does not prevent them making use of the Dark Gods and their servants as and when it suits them. They seem to regard their relationship with Chaos as a kind of business affair; a dynamic of mutual benefit. As such, it is possible for Night Lords to gain Chaos rewards, mutations, daemonhood etc, and for them to fight alongside the likes of Plague Marines, Khorne Berserkers etc. They simply regard the Chaos Powers differently from, say, the Word Bearers, who worship them as Gods. Instead, the Night Lords seem to regard them as extremely powerful allies whom they can use and abuse as they see fit.

 

However, it's entirely possible within the elasticity of the background to create a Night Lords splinter force that has fallen under the sway of one Warp Power or another. I imagine that they are particularly susceptible to Tzeentch, given their sorcerer's prediliction for precognisance, and to Nurgle, given their general melancholia and "woe is me" attitude.

[Lord_Caerolion]

Or to Khorne, for revelling in their sadism and bloodlust, or to Slaanesh, for loving the sensations of fear and panic too much...

[Kol Saresk]

Or even Malice for being the devilish rogues that they are.