Thaddeus was one of a pair of Metathran commanders created by Urza as part of his Bloodline project to lead the Dominarian armies during the Phyrexian Invasion. He was the twin of Agnate and the two shared a semi-telepathic bond, which helped coordinate their armies on the battlefield.
After they were created, instructed, and trained, Agnate and Thaddeus were placed in the Curtains of Time, time bubbles on Tolaria where time passed extremely slow. Years later, but seconds from their point of view, the Invasion began. Urza and Barrin released the two commanders, but Agnate collapsed from the stress. Thaddeus used CPR to revive him, but to Urza's surprise he did so not because he just wanted to save a useful ally, but because the two commanders felt a strong connection. Thaddeus even said he could not fight without his "brother".
After this incident the two were transported to Koilos, where the most important battle of the first stage of the Invasion was being fought. Agnate and Thaddeus each led part of the army in a pincer attack against Tsabo Tavoc's Phyrexians that had taken the caves. Unfortunately, Thaddeus's bond with Agnate was intercepted/interrupted by Tsabo Tavoc, who crafted a plan to lure him into her trap. While Agnate's division of the Metathran army became mired in battle, making little progress towards the Caves of Koilos, Thaddeus was able to easily cut a path through the Phyrexians to the Caves, wherein he was captured and then experimented upon by Tsabo Tavoc. Her cutting and probing into his body confirmed for him what he had already begun to suspect—that the Metathran and Phyrexians were more alike than different, the main difference simply being that they had been created by different masters. At the end of the battle, he was found by Agnate. Only his ribcage and head remained. Agnate hoped Urza could build a new body for his brother, but, because of what had been confirmed for him, Thaddeus no longer wished to fight for Urza. Agnate then fulfilled Thaddeus' last wish, delivering a coup-de-grace to him to end his suffering.
Thaddeus was replaced by Eladamri as the commander of the other half of the Metathran army during the Battle of Koilos.