The Afriel Strain is sometimes called as the 'Ultimate Human' via a shadowy experimental process made from the Mechanicus which used genetic material from a number of the Imperium's greatest heroes to make unlimited clones of them. Examples incude the legendary Macharius being among the individuals replicated in this manner.
While the idea of an entire army composed of Macharius, Sly Marbo, Straken, Ciaphas Cain and Yarrick may sound Awesome to the point of being a borderline Mary Sue. The Strains themselves...seem to suffer a severe case of 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 - followed by a rapid decline and destruction of the regiment. It got so bad to the point that some Mechanicus adepts believe that their cloned genetic makeup may interface with the Warp in some way to alter the laws of probability. Why the regiments commit acts of mass death by unknown mishap is still officially unknown. 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.
Two show up in a Last Chancer's Novel, where they were portrayed as being pretty good at killing Nids but bad at interacting with humans. They, essentially, were attached to the unit to see if how bad things go. Here, their Mary-sueness is downplayed. While pretty prodigious at making things dead, they lack any sort of empathy or social understanding. One of them, a female, shows up to mess topless, unaware she is surrounded by penal legionnaires (or just so confident in her skills she didn't care). This ends with someone trying to grope her and, with an apparently swift motion, her ripping out his eye from the socket and nonchalantly handing it to the commander. At least they aren't psychopathic. Both end up dead, as per obligations (as far as we know), but one simply doesn't make it to the landing zone when they have to escape and the other basically dislocates a sergeants shoulder when she tries to go out of the dropship to find him, before going out and vanishing. Which demonstrates that while they lack social skills and empathy towards others, they seem to retain their devotion to their comrades which was often a legendary part of their genetic "donors".
Afriels are normally mostly albino, with pale alabaster skin, pale hair, and colorless (not red) eyes.
One must wonder, though, why they are not trained and fielded in the capacities their legacy was famous for. Training and deploying a clone of Lord Solar Macharius as a guardsman is pretty stupid when he's famous for being a brilliant strategist as a general and Warmaster.
You can try fielding them via using proxies and changing their rules to They Shall Know No Fear, It Will Not Die and Rage as well as a strong initiative and leadership checks. Although it could be off set with a severe case of Imperial Plasma Weapons, as in they will have a chance to suddenly drop dead...for no reason.
If you're looking for something a little more authentic, but a little out of date. Afriel Strain guardsmen were given rules in Chapter Approved back in March 2005. For 5 points per squad, you could give them And They Shall Know No Fear, however to represent their horrible luck, every unit in the enemy army gets Preferred Enemy while attacking them. Furthermore the army cannot use Ministorum Priests or take Inquisitorial allies.
A Mary Sue of such untold proportions that it loops all the way back to fail.