Karn is a mysterious planeswalker made of metal, who can use all five colors of mana.
Father Karn was a metal golem created by Urza during the Phyrexian Wars. He was crafted of silver and powered by a Phyrexian heart stone. His purpose was to go back in time using the time machine Urza and Barrin built at the first Tolarian Academy to stop the Thran from ever becoming Phyrexian in the first place. However, after the machine exploded when Karn was sent back to stop Kerrick, Urza found other uses for him. Much of Karn's early life was spent traveling with Urza and building many alliances for the coming invasion. After Phyrexian incursions on Dominaria using the artifical plane of Rath, Urza sent Karn away from Benalia where he had been staying to collect parts of the Legacy weapon. When he left, he was given Gerrard Capshan to watch over and take care of after much of the Capshen bloodline was killed by Phyrexians. Later parts of his life were spent travelling with Sisay and her crew on the Weatherlight, where he partook of their adventures and proved to be a valuable asset. Gradually, Karn learned of his true purpose in Urza's designs: as the core of a very powerful artifact called the Legacy weapon. During the final Invasion, Gerrard, Urza, and Karn all formed part of the Legacy weapon to destroy Phyrexia. This sacrifice by Urza led to Karn gaining the weakstone and the mightstone and becoming a Planeswalker.
Absorbing the essences of both Urza and Gerrard gave him the "spark" of a planeswalker. In his own quest for knowledge and understanding he created the plane of Argentum later known as Mirrodin and left as its caretaker the powerful and evolving artifact called the Miriari. Karn continued his travels through the Multiverse, even taking on the young planeswalker Jeska as an apprentice of sorts. When he would return to Mirrodin, he found that he could not.
While he was away, the Mirari had evolved into the unstable and maniacal Memnarch. Under normal circumstances Memnarch held no such power to bar Karn, but due to the fracturing of the Multiverse and the resulting planar rifts (as detailed in the Time Spiral story arc), his insanity imposed a barrier around the plane that Karn could not penetrate. It was only with the death of Memnarch, through the efforts of Glissa Sunseeker, that the barrier disappeared and Karn could return to Mirrodin to try and set things right.
Disturbed by the power and instability that Memnarch dispalyed, Karn begins a quest find out the cause. His efforts return him-along with Jeska-to Dominaria, where he finds the rifts are at their worst. There he meets his old friend Teferi, who has been struggling to save Dominaria and the Multiverse from destruction. After brief council with Teferi, he journeys back in time to heal the rift at the Tolarian Academy, which is both spatial and temporal, and therefor not mendable in the present. After giving up his spark to seal the rift, the Phyrexian oil in his heartstone began to corrupt him. Until this time, the power of his Planeswalker spark had been the only thing protecting him. At this instant of horrible revelation, Karn threw himself into the Blind Eternities and locked himself away in a plane that is his alone.