From the introduction: They came from the core. The Glistening Oil’s corruption had settled deep within the plane of Mirrodin, twisting the minds and bodies of those it could reach, hiding out of sight, marshalling their forces in their search for perfection, their “Great Work” Abominations of metal and flesh, driven once by a singularity of purpose, were imbued with the essence of Mirrodin’s 5 suns, and what was once a singular entity in a different plane split into 5 factions. The native Mirrans, with their powerful artifacts and brave warriors, waged war on the Phyrexians.