The Falleen were a repto-mammalian sentient species who had a distinctive alien appearance. They were distinguished by their mottled green skin, ridged skulls, and their long, black hair which they typically wore in ponytails. Falleen exuded powerful pheromones which they used to attract mates.
Falleen were native to the planet Falleen, but were a colonial species who had migrated and adapted to dominate many worlds. and so also inhabited Ord Mantell alongside a human population.
During the Clone Wars, a cabal of Falleen nobles led the Black Sun criminal syndicate, which flourished while law enforcement declined as the Jedi focused on battling the Confederacy of Independent Systems. Black Sun became notorious in the lawless Outer Rim Territories and elsewhere, and its Falleen leadership operated out of a well-defended fortress on Mustafar. Xomit Grunseit led the cabal until Maul ultimately usurped control of Black Sun, and installed the leg-breaking lieutenant Ziton Moj as its ruler.
Black Sun was also involved in an underground dispute with the Pyke Syndicate, which led to Asajj Ventress and Quinlan Vos investigating a kidnapping by the Pykes. Ventress coerced a Falleen guard on Mustafar, leading him to believe his powerful pheromones had worked on her, and she and Vos strong-armed him into giving up information on the Pykes. The pair infiltrated the Black Sun fortress, fighting more Falleen to reach the kidnapped family.
During the reign of the Galactic Empire, Falleen like other alien species such as Umbarans and Neimoidians, were typically the butt of running jokes at Imperial Academies including the Royal Imperial Academy on Coruscant. At this time, the Falleen Ango Croom led the Varluk Organization. A Falleen gangster named Annaz was a member of the Malandro syndicate before she was attacked and tortured by the bounty hunter Brand, whose parents Annaz had formerly killed. A number of Falleen worked as overseers in MaDall's enslaving operation on Oon, and treated Sabine Wren roughly when she posed as a slave to infiltrate the operation. She inspired an uprising against the slavers, and the Falleen guards were attacked and overpowered.
Falleen first appeared in "Eminence," the fourteenth episode of the animated series Star Wars: The Clone Wars, which aired on January 19, 2013. Within the first installment of the Prequel trilogy, Star Wars: Episode I The Phantom Menace, micro-machine toy figurines of Prince Xizor, a major Falleen antagonist from Shadows of the Empire, were used in the Mos Espa Grand Arena. Because this "appearance" was only used to fill-in the background, it is not known if the cameo would be considered an actual appearance of the species. Although Star Wars: Galaxy's Edge: The Official Black Spire Outpost Cookbook brought the character into the canon continuity, it did not confirm if he was at the Boonta Eve Classic.
In Legends continuity, the Falleen species first appeared in the novel Shadows of the Empire, which was written by Steve Perry and published in April 1996.