Glacian

Glacian was an ancient Thran artificer who designed many of the Empire's most powerful artifacts, including the powerstones on which the Thran technology was based.

After being stabbed with a powerstone by Gix, Glacian caught the disease phthisis, which was caused by long exposure to powerstone radiation. When Thran healing magic only made the disease worse, his wife Rebbec used her influence in the council to recall the healer Yawgmoth from his exile.

Yawgmoth discovered the cause of phthisis and even created a serum that stopped the disease, but for some reason it had no effect on Glacian. His case only grew worse, as he developed a multiple personality disorder. He became incredibly paranoid and started to think of everyone as traitors, for Yawgmoth managed to become one of the most powerful people in the Empire.

Yawgmoth had fallen in love with Rebbec, and was trying to marginalize Glacian, banishing him to the Caves of the Damned. Trying to look good for Rebbec, Yawgmoth did remove the sliver of powerstone that had been keeping Glacian ill form his old wound, but he replaced it with the two halves of the powerstone Dyfed had used to create a permanent portal between Dominaria and Phyrexia when Rebbec wasn’t looking. The radiation of these stones killed him, but his essence was trapped inside them. Finally being able to think clearly again, he convinced Rebbec to unify the halves and use them to close the portal to Phyrexia forever.

But millennia later, two brothers, Urza and Mishra, broke the stone. They each took a half, calling them the Mightstone and the Weakstone. The brothers got caught up in a war that placed them again one another as leaders of different countries, the so-called Brother’s War, which ended with Urza activating the Golgothian Sylex, killing both brothers and decimating an entire continent.

But Urza ascended into planeswalkerhood, and the two stones became his eyes. Because of these eyes he had abilities that made him unique among planeswalkers. Blaming Glacian’s old enemy Yawgmoth and his Phyrexian followers for his brother’s death, he set out to defeat him once and for all.

Four thousand years later, at the pinnacle of the Phyrexian Invasion, Urza had Gerrard remove the stones from his sockets and place them in Karn’s chest. This completed the Legacy, and almost destroyed Yawgmoth. Karn ascended, and from then on the stones, still carrying Glacian’s essence, reside in the chest of Karn Planeswalker.

Glacian possessed the planeswalker's spark, but it was never triggered.