The "Brotherhood of Aliens" was a group of prisoners at the Rura Penthe gulag in the late-23rd century.
In 2293, when Captain Kirk and Doctor McCoy arrived at the prison, after having been framed for the assassination of Klingon Chancellor Gorkon, a large, threatening inmate demanded their allegiance to the Brotherhood of Aliens, a condition to which Kirk swiftly agreed, recognizing that he was not being given a choice in the matter. (Star Trek VI: The Undiscovered Country)