What Primarch could the Necrons have?

[Asymmetrical Xeno]

I would like to think that it's Arthor Amhrad, the Chapter Master of the Astral Knights, who all died destroying the Necron World Engine. That story always inspired me and I would like to think that at least one of them made it out alive, even as a prisoner.

This. I think this is the most interesting and entertaining possibility. I like to think he saw some horrific experiments deep in the bowels of the World engine that explain his contorted face.