The galli were the priests of the goddess Cybele, the Great Mother, and her consort Attis, who marked their entry into the priesthood with ritual self castration, an act ancient sources describe as performed in a state of ecstatic frenzy during the cult's annual Day of Blood commemorating Attis's own self mutilation and death. Having taken this step, a gallus adopted women's dress and ornament, let his hair grow long, and lived permanently outside the ordinary categories of Greco Roman gender, a status ancient writers such as Catullus, in his poem on Attis, and Lucian both describe with a mixture of fascination and unease.
The priesthood was organised hierarchically under an Archigallus, the chief gallus of a given sanctuary, and galli were known in the ancient world for public processions in which they danced ecstatically to drums and cymbals, sometimes cutting themselves further in public displays of devotion that ancient observers found simultaneously compelling and disturbing to watch. Ancient authors also describe galli using distinctive ritual instruments, including cymbals, tympana and a double piped flute associated specifically with the cult, whose sound accompanied the ecstatic dancing that marked their public processions through Roman streets.