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Karn is one of the oldest characters in Magic: The Gathering. Created by Urza, Karn ultimately became the world's first artificial Planeswalker, and he is currently the only colorless Planeswalker outside of the spirit dragon Ugin.
Karn has done a lot in his life, from the early days at the Tolarian Academy to helping the Gatewatch in their fight against Nicol Bolas. He is a master with artifacts, capable of seeing how they function by simply holding them. Karn can easily create his own artifacts, including the creation of an entire artificial Plane named Argentum, which would go on to become Mirrodin and later, New Phyrexia.
Creation And The Weatherlight
After the Brothers' War, Urza created the Tolarian Academy and invited scholars and mages from all across the land to study there. It was here where Urza began to study the past, creating a silver golem that let him travel back and alter it. This silver golem was no other than Karn. Urza gave him life when he injected the hearthstone, which made him like a person, with his own thoughts, emotions, and memories. While Urza worked on other projects, he entrusted the care of Karn to the future Planeswalker Teferi, who was an early student at the Academy.
Teferi treated Karn poorly, even giving him the name of "Arty Shovelhead." Eventually, Teferi was removed from his position when Jhoria noticed how he was treating Karn. Jhoria was also the one who gave Karn his name.
Jhoria became Karn's closest friend. However, Jhoria would be taken over by a Phyrexian and converted into a sleeper agent. Desperate to save his friend, Karn used Urza's time machine to go back in time and save her. While he succeeded, the machine would malfunction, destroying the academy and creating temporal rifts all around it.
Jhoria retired from Tolarian Academy, opting to return to Shiv to run the Mana Rig. This sent Karn into a depression, his only friend gone. In an effort to distract himself, Karn joined the Weatherlight crew. This did not help, but Urza, at the request of Barrin, made it so Karn could not recall memories beyond 20 years. Karn did not fight this, but he would look at a picture of Jhoria each night while saying "Jhoria is my friend" so that he would never forget her.
Spark Ignition
Karn was a key of the Legacy Project, Urza's plan to ensure he would be able to decimate Phyrexia. It consisted of Karn and multiple artifacts of Urza's creation. When collecting the parts of the Legacy, Karn was tricked into killing an innocent and being frozen. When he was reactivated by Gerrard, Karn became a pacifist, vowing to himself to never kill again.
Karn ultimately reclaimed the Legacy from the Sliver Queen, explaining that the Legacy was a part of him just like the Slivers of Dominaria were a part of her. This allowed the Weatherlight crew to have access to the Legacy when Phyrexia initiated the invasion of Dominaria.
While in a fight in the Caves Of Kolios, Karn's mental block would be shattered, and memories of Phyrexia killing Jhoria back on Tolaria returned to him. This caused him to cease his vow of pacifism and attack the Phyrexian Tsabo Tavoc, helping to kill her and save Gerrard.
During the final confrontation with Yawgmoth, the Legacy was activated by placing the Mightstone and Weakstone, the eyes of Urza into Karn. This caused a giant explosion of white mana to burst from Karn, decimating Yawgmoth and the Phyrexians, with both Gerrard and Urza sacrificing themselves in the process. Karn survived, inheriting his creator's spark and ascending into a Planeswalker.
Argentum And Phyrexian Corruption
After the victory over the Phyrexians, Karn would later create Argentum, an artificial Plane. Karn resided here and sent probes out to Dominaria to monitor it. This led to the creation of the Mirari. Karn would eventually turn it into a golem like himself, which became the Memnarch.
Karn left with recently ascended Planeswalker Jeska. However, due to the Phyrexian artifacts, the Mightstone and Weakstone inside of him, as well as the hearthstone, he left traces of Glistening Oil on this Plane and in any other he visited.
The Memnarch became exposed and corrupted by the Glistening Oil, which drove it insane. It renamed the Plane Mirrodin. The Memnarch was ultimately stopped and returned to the Mirari, where it was kept in the care of locals of the Plane that helped Karn stop it.
Jhoria called Karn and asked him to return to Dominaria to close the time rifts that were still present from the incident at Tolarian Academy. He succeeded in going back in time to seal the rift during the invasion, but he would lose his Planeswalker spark in the process. Without it, he was prone to the Phyrexian artifacts inside him and began to be corrupted by them.
Karn retreated back to Mirrodin, which had become New Phyrexia. The Phyrexians of the Plane worshiped him as the Father Of Machines. All of the Praetors of the Plane tried to manipulate him for their own goals, with Sheoldred seeking to kill him and take his place.
Karn managed to call out to Venser and lead him and his party to him. By the time Venser and his ally Melira arrived, the Glistening Oil had nearly completely corrupted Karn. While Melira was able to cleanse most of the oil from him, she was unable to take it out of Karn's heart. Venser, knowing he was dying, sacrificed himself to fully cleanse Karn and give him his spark, curing Karn and giving him a spark once more.
The War Of The Spark And New Phyrexian Invasion
Karn returned to Dominaria in search of the Sylex, the artifact Urza used to destroy Argoth and end the Brothers' War to destroy New Phyrexia. It was here where he met Chandra and was recruited by the Gatewatch to help in the fight against Nicol Bolas.
During the War Of The Spark, Karn used Ral Zarek's goggles to create a spell so that he and his fellow Planeswalkers could see their auras and know who were Planeswalkers —many of them had never met before. When the Planeswalkers split up, Karn was a part of the team that traveled to Amonkhet to shut off the portal to Ravnica. Karn was the one who shot a drone into Tezzeret's chest, closing off the Planar Bridge and shutting Amonkhet access to Ravnica off. After being given the God Hazzoret's spear, Karn returned to Ravnica and delivered it to Niv-Mizzet.
Sometime after the War Of The Spark, Karn returned to his goal of destroying New Phyrexia. To learn more about the Sylex, he travels back to the Caves Of Koilos where he discovers Sheoldred being healed by Rona, an artificer who was a devoted follower of the Phyerxian Gix. She helped in trapping Karn in the caves, preventing him from warning the others that Phyrexia had arrived on Dominaria.
Karn was eventually saved by fellow Planeswalker Ajani months later, and he worked with Jaya, Jodah, Teferi, and many other Dominarians to fight off the invasion. However, the Phyerxians had already converted many into sleeper agents, with the goal of learning where the Sylex was so they could destroy it.
Karn would give different locations to all those who asked in case anyone had been turned into a sleeper agent, making them have false information. This worked on Stenn, who acted with a false location and was defeated by the group. However, on the Mana Rig, the Sylex was revealed, activating the compromised Ajani, revealing himself to be compleated. Ajani crushed and destroyed the Sylex and disabled Karn, kidnapping him and leaving Dominaria with Sheoldred, taking Karn back to Elesh Norn. Back on New Phyrexia, he was restrained and imprisoned with Elesh Norn, who told him that "It is the beginning of great things, Father. The beginning of everything."