From Halopedia, the Halo wiki
Blackdust was a notorious "courier guild" of off-world smugglers based on the human Outer Colony of Aleria. The guild was particularly against the Mols'Desias mining union, believing the union forced Alerians into lives of crimes by its monopolization of the job market with little competition in the planet's failing economy.
The Blackdust guild was responsible for murdering Tarek Boesis, the head of Mols'Desias and the grandson of the union's founder, in his home. Claiming to be culpable for the crime, the guild believed the murder was justified, as it—in their own words—served "to bring retribution to those who were responsible for the silent deaths of countless thousands..."
List of appearances