The Plane of Argentum was created by Karn sometime after the invasion of Dominaria. Karn had become a planeswalker and used his abilities to make his very own plane, a plane devoid of organic life and populated only by artificial life.
Karn was happy with his plane, but after a while felt the wanderlust that planeswalkers often do. Deciding his plane needed a protector, he created the gave one of his many golems, Memnarch, another of his creations, the Mirari.
The Mirari was originally designed to be a probe, Karn sent it to Dominaria to keep an eye on it, but the Mirari ended up causing wars over its power. Finally it was recovered by Karn, and eventually installed into Memnarch.
Once Karn had left Argentum he named Memnarch his steward, giving him full power over the plane. Memnarch quickly started to go mad. Karn had inadvertently left behind Phyrexian oil, from his own body in his palace. The oil drove Memnarch insane.
Memnarch became obsessed with the power his master had, and became oriented to a single goal… become a planeswalker. However this posed problems to the golem. First of which was that artificial creatures could not be born with the planeswalker spark, so even if he had the means to transfer the spark from one being to another, there would never been on on Argentum.
The decision was clear, Memnarch must change the plane itself to cultivate a spark. And so Memnarch did. He made the plane more inhabitable to living orginisms. Once his terraforming was complete, he started destroying his fellow golems that Karn had created and named his new plane Mirrodin.
Memnarch then set to stealing living organisms en mass from other planes with devices he had created. He became more and more powerful, even powerful enough to keep Karn from being able to return to his own plane. Memnarch spent much of his time in his new lair, the Panopticon where he could observe the plane and seek out potential planeswalkers.
Finally Memnarchs efforts were successful and he found an elf that had the spark, named Glissa. Memnarch attempted to steal her spark, which ultimately ended with his death. After Memnarch was destroyed, Karn was able to return to his plane.
After the events in Time Spiral Block, Karn sensed his mind being corrupted. He retreated back to his plane to recover and went to the very core of Mirrodin. Karn started to loose his grip on his sanity and the Phyrexian nature of his powerstone started to corrupt Mirrodin.
Mirrodin’s populace was nearly helpless to stop the corruption of their world. Most fought the New Phyrexian beings, but the started to overrun the resistance. The New Phyrexians worshiped Karn as the father of machines and made every effort to transform the plane in his name.
Karn was eventually saved by his friend and fellow planeswalker Venser, but it was too late for him, and Karn had been fully transformed into a Phyrexian. Melira was able to cleanse Karn’s body, but his heart was too corrupted.
Venser decided to make the ultimate sacrifice for his friend. Venser was deteriorating because of phthisis, a degenerative disease caused by over exposure to powerstones. Thus Venser knew he would die, so he gave all his remaining mana, his life force and his spark to Karn.
This saved Karn and killed Venser, but Mirrodin was no more, only New Phyrexia remained.