Steel Plague

The Steel Plague is a reactionary evolution of the Brotherhood of Steel, an overzealous, techno-religious dictatorship that would have formed in New California if Rhombus died prematurely and the order found itself under new leadership. The fanaticism would result in it devastating the fledgling New California Republic in 2186, starting a new Dark Age that could last a thousand years.

Behind the scenes
The Steel Plague will only emerge if the player character kills Rhombus in the game, preventing his ascension to the position of high elder and resulting in new leadership. The canon outcome is that the Brotherhood instead becomes a major research and development house, remaining outside the power structures of New California under his rule.