Glacian was an ancient Thran artificer who designed many of the Empire's most powerful artifacts, including the powerstones on which 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 a traitor, 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 the powerstone that had been keeping Glacian ill from his old wound, but 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.