I'm building a chaos marine army, and the basic idea is thus:
Two forces are lured onto Hope's Demise by the machinations of the Chaos Gods. A band of Death Guard, and a band of World Eaters are caught in the subtle net of Nurgle’s humor.
In a prank of horrific proportions, Nurgle gives both a disease that affects not the body; instead it affects the mind and the memory in particular. Their minds are wiped, and they believe themselves back in the time of the Heresy. And they believe themselves to be heroes. They hallucinate, so that they believe that they are without mutation, their loyalist foes are horrendous mutants, and their defilers and daemon allies are dreadnoughts and loyal militia respectively. Which explains why they are in their original Legion Colors….