The Dark first appeared when explorers broke a seal on the outermost planet of our solar system. After Nero was breeched, the dark appostles were allowed entry, or maybe just became aware of man and a new race to corrupt and assimilate.
The Dark Legion is an evil force of alien inteligence. Completely foreign to man's understanding, the Dark Legion use alien tech and mystical powers of the Dark Symmetry. They animate the bodies of dead soldiers and force them to battle against former comrades. They are capable of traveling between planets and solar systems, and seamlessly merging flesh and machine.
The Dark Legion are the most dangerous force mankind has ever encountered as it works tirelessly, endlessly, to corrupt the tech, minds, and bodies of man.