The Circle of Steel is a group of hardliners within the Brotherhood of Steel who function as the group's Internal Affairs and is one of the four main factions in JesusKong333's Fallout: Detroit.
In 2254, the Elders on the West Coast sought to reestablish contact with the Midwestern chapter and sent a contingent of soldiers to locate them. This group, led by Owyn Lyons, was unsuccessful and went on to form the East Coast Brotherhood of Steel. Thirty years later, in the wake of the NCR-Brotherhood War, the Elders have decided to seek them out again and bring them back into the fold. The West Coast Brotherhood suffered heavy losses during the war and their last reports showed the Midwestern chapter controlled a vast robot army from Vault 0 under Cheyenne Mountain, a robot army that could be used to bolster the West Coast's ranks. The group chosen to find them was the Circle of Steel, the Brotherhood fundamentalist group that acts as Internal Affairs. The Elders knew the Circle would not abandon their mission as Lyons once did and no one was better suited to judge whether the Midwestern chapter still held to Brotherhood ideals.
The Circle began their mission seeking out Vault 0. Upon arrival, they found Cheyenne Mountain a smoldering crater. Pushing further east, they discovered town after town under siege by robots with Brotherhood of Steel logos. The Circle was appalled this was the fault of the Brotherhood. The Brotherhood teaches the Great War happened due to man's misuse of technology. This was everything the Brotherhood stood against. As the Circle continued their search, word eventually reached them of a group that established dominance in the ruins of old Detroit, a group in power armor with advanced weaponry and an army of robots. They made their way to the Motor City Wasteland and couldn't believe what they found. The Midwestern Brotherhood had declared their Inquisition on the locals of Detroit. Their robots were monitoring citizens at all hours. Checkpoints had been set up and the city locked down, no one in or out. Those that spoke out against the Brotherhood often went missing. All misuses of technology from the Circle's fundamentalist viewpoint. The Midwestern chapter had fallen far from the ideals of the Brotherhood and would need to be eliminated.
Thus, the Circle of Steel have begun their campaign to eradicate the Midwestern chapter. The Circle has been gathering intel on the Brotherhood's operations. They've attacked Brotherhood patrols and extracted information from the survivors and Circle infiltrators are posing as new Brotherhood recruits to gain access to Fort Wayne and Zug Island. Recently, one of their infiltrators posed as a local speaking out against the Brotherhood in order to gain access to the Brotherhood's prison. When he had gathered the intel they needed, the Circle orchestrated an explosion at the prison to extract their man. Soon they'll be ready to leave the shadows and strike, perhaps by aiding and fighting alongside the Resistance.