The silver golem Karn is a Planeswalker who can wield all five colors of magic. He was created to endure time travel and to serve as the key to the Legacy.
Over a thousand years ago, the Planeswalker Urza created a new weapon in his war against Phyrexia: the silver golem now called Karn. Urza's plan was to send his creation back in time to when Phyrexia was a nascent threat. The plan backfired and the island of Tolaria was devastated. But Karn had many other purposes to serve.
The golem was also the first artifact of the Legacy, a collection of objects whose arcane might would eventually be used against Phyrexia. He was given the heartstone of the rogue Phyrexian sleeper agent Xantcha, once Urza's ally and companion. Centuries later, it was Karn who rescued the human component of Urza's Legacy from Phyrexian raiders: Gerrard, who would become captain of the Skyship Weatherlight.
Decades after, when Phyrexia invaded the plane of Dominaria, Karn fulfilled his destiny by taking his place within the Weatherlight hull, surrounded by the artifacts of the Legacy. The ship became a weapon, delivering a devastating blast of white mana that destroyed Phyrexia's "god," Yawgmoth, the Father of Machines. At that moment, Karn became a planeswalker, inheriting the Planeswalker spark from his creator.
Karn later created a plane of his own: a metal world he called Argentum. When he brought the Planeswalker Jeska to see his plane, he introduced a contagion from Dominaria to his world—a Phyrexian substance to which, as a Planeswalker, he was immune and unaware. As Karn explored the Multiverse, the Phyrexian contagion took hold, reshaping the plane and corrupting the mind of the Warden that Karn had left to watch over his creation. Even when Karn returned to Argentum, now called Mirrodin, and set things right with his world's inhabitants, the contagion continued its quiet work.
During Dominaria's temporal crisis, Karn was called on by his friends Jhoira and the Planeswalker Teferi, both of whom were once students at Tolaria. Together with a new ally, Venser, they worked to prevent time itself from collapsing by finding those who were willing to surrender their Planeswalker sparks to heal the temporal damage. Karn accepted the task of healing the greatest of these time rifts: the one he had helped create at Tolaria. And although he succeeded, at that moment he became aware of—and susceptible to—the contagion he carried. In his last moments as a Planeswalker, he hurled himself through the Multiverse to Mirrodin, instructing his friends not to follow.
His mind addled and broken by the Phyrexian corruption, Karn found himself inside Mirrodin amid a fully fledged Phyrexian ecosystem. The contagion had caused Mirrodin to grow tendrils into the plane's core of raw mana, vastly accelerating its Phyrexian metastasis. Karn was hailed as the next Father of Machines by the new Phyrexians.
The Planeswalker Koth, a native of Mirrodin, sought allies to help him save his world. He recruited Elspeth Tirel as well as Karn's Dominarian friend Venser. When the trio arrived on Mirrodin, they were forced to acknowledge that the world was poisoned beyond saving. In the end Venser sacrificed himself to transfer to Karn his Planeswalker spark as well as his immunity to the contagion, in hopes that Karn could prevent the corruption from taking hold on other worlds. Grief-stricken by the fate of his world and the death of his friend, Karn set out into the Multiverse again, retracing his steps to find the Phyrexian oil and render it inert.