The Grey Men

Once a race of genetically engineered killers and slaves marketed out en masse to the highest bidders, The Grey Men rebelled after the birth of their leader, The Grey Emperor.Leading his kind in revolt, they massacred the entire population of the mining planet Thanatos VIII, and began to spread their conquest further.

Machines of Flesh
First created by the Monsoon corporation in 2120, Grey Drones where made from mechanical organs and flesh grown in chemical chambers, named for the grey coloration of their artificial skin. Grey Drones didn't fall under any labor law due to their artificially created genome not being considered humanoid. The loophole was exploited by hundreds of massive companies, all while the Greys where born into a a world apathetic to their pleas. Any rebellion was quickly put down by the kill-switch embedded in their heads

The Thanatos 8 incident
While on the Akatalium mining planet Thanatos VIII, a single Grey Drone known as The Grey Emperor was born and sold with a fatal flaw. The complete absence of a kill-switch. The Emperor was disgusted by what he saw. Drones sent down into unsafe tunnels, getting their arms crushed under rocks just to have the wound cauterized and be sent back down. He began to gather spare mining tools, cobbling together makeshift weaponry. Soon executive offices near his site began to go up in flames in the night, heavy machinery would break down and kill any nearby human operators. And in secret, The Emperor honed an ability explicitly forbidden, control of Phalek-Phos. After a failed assassination attempt towards the CEO of the Monsoon corporation during a visit, he was set to be executed. As the engineer activated the kill-switch code, he found it was no longer functioning. The Emperor used his control over Pyro to escape and kill every on-site official who could activate the kill-switch. With an army of highly durable soldiers at his side, The Grey Emperor went on a genocide of every human on the planet at the time.

The Conquest
After seizing a Grey Drone production facility, The Emperor raised an army and began a plan to conquer the entire Necros system, believing that the system was "built on the backs of the Grey Men" and thus belonged to them. He sent ships out throughout the system, raiding store houses for resources and liberating laborers. They came into direct conflict with The Prismarch after they raided a lab on Thanatos VIII's moon, Abyssum in search for a captive scientist.