The Afriel Strain was a shadowy experimental process which used genetic material from a number of the Imperium's greatest heroes to, in effect, make clones of them. The legendary Macharius was among the individuals replicated in this manner. This created warriors who were mentally superior to any regular Guardsman, albeit prone to disaster.
The Afriel Strain was an attempt to create the perfect soldier, though it resulted in unforeseen mishaps. For no reason that their creators could ascertain, the Afriels appeared to attract the animosity of other warriors and suffer from the most appalling bad luck. In each case when the Afriel Strain has been fielded, it has met with a period of success for its soldiers are undoubtedly superior. This superiority is not particularly in physical terms, but mentally, as they show absolutely no fear in combat. This initial period of success was always followed by the rapid decline and eventual destruction of the regiment. Nonetheless, whoever the creators of the Afriel Strain may be, they continue to experiment, fielding their "perfect soldiers" across the war zones of the Imperium.
Afriels are universally albino, with pale hair, alabaster skin and colorless eyes, something that only provokes the hatred of others.