The Steel Plague is a possible reactionary evolution of the Brotherhood of Steel which ultimately did not come to be.
This group is defined as an overzealous, techno-religious dictatorship that would have formed in New California if Rhombus died prematurely and the Brotherhood found itself under new leadership. Its fanaticism would have resulted in it devastating the fledgling New California Republic in the 2180s, starting a new Dark Age that could have lasted 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, which has been shown to be non-canon by later titles. The canon outcome is that the Brotherhood instead becomes a major research and development entity, remaining outside the power structures of New California under Rhombus' rule and allowing the NCR to develop.
- Fallout endings: "The Brotherhood of Steel, under new leadership after the death of Rhombus, becomes an overzealous, techno-religious dictatorship. In 20 years, the Steel Plague devastates the newly formed New California Republic, and starts a Dark Age that could last a thousand years"