The Brotherhood of the Knights of the Black Diamond, often shortened to the Brotherhood of the Black Diamond, is a secretive sect descended from the Knights Templar and connected to the Vatican. It is charged with the protection of many powerful artifacts, one of which Artie desperately needed in 'A New Hope' to restore the Warehouse, but the use of this object will unleash an incredible evil which is why the Brotherhood protects so devoutly. The Brotherhood will do anything to achieve its goals. They are gifted fighters who are willing to go to their deaths, if necessary.
The HQ of the Brotherhood is located in Richerenches, France, which is a village famous for its Perigord truffles, a.k.a. black diamonds. The public cover for the Brotherhood is they collect and auction truffles for the church.
Throughout season 4, Artie was convinced Brother Adrian, the leader of the Brotherhood, was trying to force him to undo what he had done by activating the Astrolabe a second time. Brother Adrian would steal artifacts Artie had snagged from the Warehouse and release them into the world thereby undoing Artie's life's work. Brother Adrian threatened he wouldn't stop stealing artifacts until Artie activated the Astrolabe. Brother Adrian went so far as to send dangerous artifacts like Ignaty Gryniewietsky's Tattoo Box to Pete, Myka, and Claudia's loved ones just to threaten Artie.
Eventually Mrs. Frederic and Steve went to the Brotherhood's HQ to stop Brother Adrian once and for all. However, when they arrived they found the place abandoned and that the entire Brotherhood, including Adrian, were all imprisoned in a painting by Rembrandt's Frame which Artie most likely sent them.
The Brother Adrian Artie was seeing the entire time was just a hallucination caused by the Astrolabe and Artie was really stealing artifacts from himself this entire time.
After Artie was turned back to normal with Francesco Borgia's Dagger, H.G. Wells returned the Astrolabe to the Brotherhood.
- Brother Adrian - Leader of the Brotherhood
- Ferdinand Magellan's Astrolabe: Stolen by Artie from the Brotherhood's headquarters in Richerenches, France. Later returned to the Brotherhood by H.G. Wells
- Artie mentioned he hates running into templer knights as they're so intense and overly devoted to whatever task they've been given.