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.
On Mirrodin, the organic and the metallic have evolved together, creating life that is a strange mix of biological and artificial. From sharp razorgrass that grows fast enough to impale unfortunate humans, to oceans that flow with an unusual form of quicksilver, even the most delicate lotus flower is made of metal. The intelligent beings who walk Mirrodin are no less strange, an odd mish-mash of muscle and steel. Most don't think this is strange in the slightest.
Artifact creatures also walk the planet. Some seem to have come about naturally, while some intelligent, probably long-dead, force has undoubtedly invented others. Originally, four suns (some beings call them moons) orbited the planet. Later, a fifth sun - a green sun - joined them. The suns seem to hold some kind of power over the land below. They are worshipped and feared by Mirrodin's inhabitants.
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. He used his soul traps to bring living things from other planes. He populated the world with elves, goblins, leonin, vedalken, loxodons, humans, and more - turning the plane into his own massive, metallic vivarium. Their presence on the plane had even caused their anatomies to become mingled with metal, but they still hadn't lost the connection of heritage with their former worlds. They were borrowed, not born. And when the soul traps were triggered again, when Memnarch was defeated, the Vanishing whisked them away. They were returned to their former worlds, snapped back along those lines of belonging out of the metal plane.
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 younger generations stayed, because their identities had blended with the metal world around them. They were truly part of Mirrodin and had no other home to return to. Despite Memnarch's involuntary relocation scheme and draconian population control methods, he had actually constructed a true living world—a plane with a true ecosystem in its own right and beings that belonged there. The refugees from other worlds had, over time and reproduction, become natives. They had become Mirrans, the natural residents of an artificial world.
The Vanishing was the ultimate test of belonging. If any of the living races of Mirrodin didn't belong there, they would have disappeared off the world already. Mirrodin truly belongs to the Mirrans now. From their roots scattered among myriad other planes, they have become natives and stewards of this plane. They have a right to live and thrive under its suns.
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.
Because the world is metal-based, all of the creatures that live on Mirrodin have evolved metal anatomy. Every creature on the plane of Mirrodin is infused with metal. Goblins have spikes of iron. Wizards are part quicksilver. And the bodies of trolls are shot through with copper. This is a natural process - their metal anatomy is a natural part of their bodies.
Autonomous metallic life is plentiful on Mirrodin, from Drill-Skimmers on up to Razorfield Rhinos and Steel Hellkites. The designs of artifact creatures on the plane change over time. While many artifact creatures are created by artificers or other sentient beings, there also exist a smattering of autonomous processes that can generate them without conscious input. Artifact creatures are generated by self-sufficient foundries left over from Memnarch's time, golem-forges built into certain areas of the plane, and the activities of a few enterprising myr. As the models spun out by these processes change, this gives rise to a kind of metallic design evolution. Later models "inherit" design features from earlier models, and designs branch out to fit the environments, purposes, or predators they encounter.
Mirrodin is a spherical world, orbited by five small suns (which therefore also function as moons). Each of the suns is colored according to one of the colors of mana.
Countries and Realms of Mirrodin
Geographical Features on Mirrodin
Cities on Mirrodin