Mirrodin: a plane made entirely of metal. Here, thorny copper trees create artificial forests, great structures of iron and rust form mountains, and the blades of grass on the plains are literally blades. Five shining suns cast their light on the shining world below them, one for each color of mana. Humans, elves, and goblins dwell here, along with vedalken, leonin, and the zombies known as the nim.
Long ago, the metal plane was created by Karn, the silver golem built by Urza, as a test of his newfound power as a planeswalker. He populated his plane with golems created in his own image and also built a "Warden" to watch over his plane as he explored the Multiverse.
But something went wrong. Karn unwittingly introduced a dark contagion to his own world: Phyrexian oil. Over the course of centuries, the oil took hold on Mirrodin, changing it and its inhabitants. The Warden grew unstable, gaining sentience and calling itself Memnarch. Over time Memnarch grew paranoid and angry, feeling abandoned by his creator.
And as Memnarch's madness grew, so did his power. Using arcane devices called soultraps, Memnarch kidnapped countless living things from other planes and brought them to Mirrodin. His goal was to find a being with the planeswalker spark and take it for himself, enabling him to leave Mirrodin and find his creator.
The elf Glissa Sunseeker was the first being born with the planeswalker spark on Mirrodin. Glissa uncovered the secret of her world: that it was hollow, and that its mythical tyrant was real and dwelled inside. Through her own guile and power as well as the help of her allies, Glissa defeated Memnarch. The soultraps were destroyed, and instantaneously the first generations of Mirrans vanished, returned to the planes from which they were taken.
The remaining Mirrans were left to deal with the disappearance of their eldest forebears. Few had any idea what had transpired inside their world—only that their grandparents, elders, and chieftains were gone. When Glissa and her goblin ally Slobad returned to the surface, they were set upon by hordes of panicked goblins. Slobad fell, and Glissa retreated back into the interior, where the tiny constructs that still scurried there kept her safe, sound, and sleeping ...
Meanwhile life continued on the surface as the Mirrans worked to resume their lives. But inside the plane, the Phyrexian contagion that Karn long ago brought to his own world continued its quiet work. The oil spread, and everything changed when the mycosynth reached the mana core that still shone inside Mirrodin.