Before he was reforged he was a reformed sellsword that became a fearless war leader reputed for striking the hellish slave-cities of the Laudnalus Reach with his followers. He liberated slaves of all races and them arming them to fight back against their oppressors. He managed to stay ahead of Chaos slavers for almost twenty years before being betrayed by one of his own. He and his closest lieutenants were cornered in the Greyfall Vale and overrun. For these qualities he was spirits away by Sigmar from his doom to serve a greater cause as a Stormcast Eternal.
Lord-Celestant Sargassus led his Warrior Chamber, the Heavenhost, to war upon the Igneous Delta and Anvrok’s Hanging Valleys. Sigmar, seeing their worth, entrusted them with a mission of their own, one that require both martial might and a cetain diplomatic flair.