Scourge

Background
While the detachment of the Brotherhood of Steel under Owyn Lyons was heading west across the continental United States, bound for the Capital Wasteland, or the ruins of Washington, DC, to look for lost technology in the nation's former capital, they encountered many groups of survivors who had adapted in various ways to the post-apocolyptic conditions. One such region was The Pitt, formerly known as Pittsburgh. Before the Scourge, the Pitt was lawless, ruled by no one, with rape gangs, raiders, slavers,and many other kinds of horror all freely wandered, terrorizing the few innocent survivors.

Sweep of The Pitt
As they passed into the region, the Brotherhood of Steel, under Owyn Lyons swept through the area, easily devestating any large pockets of resistance, taking any unmutated children for the Brotherhood, such as Paladin Kodiak. The area was left in ruins, with few casualties on the side of the Brotherhood, Ashur, of course, being a notable exception. Buried in rubble, his squadmates were certain he had been killed, and continued their sweep without him. He regained consciousness after the Scourge had moved on. The stage was set for Ashur's rise to power, and subsequent control of the Pitt. This part of the story, told to the player by Ashur, is corroborated by Ashur's Power Armor, at least somewhat circumstantially. It is most likely true, as aside from the shock and awe effect with most other raiders and slaves, there is little reason Ashur would lie about his background.