At the edge of Mirrodin's metallic fields lies the toxic morass of the Mephidross, a vast swamp whose power feeds the world's blackest souls. Rumors of strange and deadly creatures emerging from the dark fen are spreading faster than its corroded borders. Few who have seen these abominations believe them to be a new order of the undead Nim. Fewer still understand that they are the harbinger of a great corruption—an impending Phyrexian invasion.

Venser—former apprentice and friend of the Planeswalker Karn, Mirrodin's creator—and his allies Elspeth Tirel and Mirrodin native Koth, know of the Phyrexian contagion and search for a way to contain it. Having first consolidated their power in the plane's hollow interior, the Phyrexians are found on every part of the surface now, from the Razor Fields to the city of Lumengrid—and larger numbers are beginning to mass around the centers of civilization.

The would-be Father of Machines, Karn, lost to creation, is the only Planeswalker who can potentially stop the Phyrexians on Mirrodin and throughout the rest of the Multiverse. He must be found.