They first came to be at the very first day of 500M41 on the war torn world of Stygia. The combined forces of Imperium and Eldar fought there against massive chaos invasion. Prize of this conflict was an ancient tech-artefact from the times of the Dark Age of Technology, though its origins were unknown the machine combined both human and eldar technology and was considered to be a powerful weapon. During the war it turned out that in fact the machine was a mind control device, it was able to create personality imprints that forced living beings to behave in predefined manner. Quite problematically a sacrifice was required for device to function, because it was determined that to create an imprint it has to first consume the mind and body of a human.
When it was discovered, Eldar Farseer leading the host wanted to destroy machine immediately, but, thanks to its self-repair capabilities, it proved to be more resilient than anybody could have expected. Time was needed to set demolition charges in a proper manner, but with chaos force closing on their positions both Eldar and humans realised that only a miracle could save them. And the miracle happened, or so they fought. Reconnaissance team found a small tribe of Blood Axes camping deep within the jungle and defenders decide to use power of the device to turn those orcs into their allies. A volunteer raised up from the ranks of Space Marine detachment, surviving logs listed him simply as "an Apostle" though his chapter of origins remains unknown, and allowed himself to be consumed by the machine. His body and mind had been shattered, but the operation has been successful - his personality has been imprinted onto the orks camping in the jungle.
Sadly stress associated with its activation proved to be too much for an ancient device, it's not exactly clear what happened, but the explosion wiped out both the besieged and the besiegers.
Confused and stunned orks woke up to the hellish storm of lightning and fire raining from the sky. Machine did its work as best as it was able to, but it was never meant to be used on such a species, and so its effects were not perfect. Parts of their old selves fought against the forced imprint, some parts were destroyed, others brought to the surface. Greenskins were hallucinating and walking in circles drooling for weeks until the biggest and most powerful of them stepped forth. Finally pieces of what was now his mind fall together giving him a clear vision of what he and his boyz had to do. The vision itself was still unclear and yet frightening, he had never thought something might be so dreadful, the twin voices have said: You gort will now fight for da'Emprah in his Waaaaagh... or all da fightin will stop, and that would be unorky. Following this command Da Apostork, as he ordered to call himself, gathered remaining boyz and set of to save the fighting from dying out.
Da Emprah Protects
After he finally departed Stygia, Da Apostork led his boyz towards the nearest colonized system. It happened to be shrine world dedicated to Saint Marcus the Blind, currently under attack by the last remnants of the same chaos host that had almost been wiped out at Stygia....