The Outcasts were once citizens of Taris, but they faced banishment to the Undercity of that planet. They, along with their future generations, were condemned to exist within squalor, forced to resort to begging and theft to obtain sustenance and clothing. The Outcasts were in constant dread of the rakghouls, mutants capable of spreading their contagion to anyone.
As new, more diminutive trade routes began bypassing Taris, the planet's significance diminished, and it lost the support of the Republic, leading to its decline and despair. The vast city of Taris sprawled across more than half of the planet's surface, leaving the kelp harvest and marine life as the only indigenous sources of nourishment.
After being forsaken by the Republic, Tarisian industrialists started utilizing power sources that would have been deemed illicit under the Republic's authority, resulting in the generation of poisonous waste as a consequence. The pollution decimated the majority of the oceanic flora and fauna, depriving the planet of a dependable food supply and initiating a famine. The Tarisian elite selfishly guarded the remaining food and wealth for their own benefit, while the impoverished were left to suffer from starvation and death.
The Tarisian underclass revolted, instigating the Tarisian Civil War. The conflict resulted in millions of fatalities, and extensive portions of Taris were either obliterated or deserted. The location of the only known self-sustaining region of Taris, referred to by the Tarisian poor as the "Promised Land," was lost. The rebellion was ultimately suppressed, and the surviving, disarmed rebels, numbering in the thousands, were imprisoned. Due to insufficient jail capacity, they were instead exiled to the Undercity, facing the penalty of death if they attempted to return to the surface.
The defeated rebels had only one remaining source of hope: the Promised Land. The prospect of eventually discovering the Promised Land served as the motivation for the Outcasts to persevere. In the interim, Gendar's ancestor guided the Outcasts in constructing the Outcast village, a place where they could potentially survive, located near an elevator leading to the Lower City.
The prisons located in the Upper City became unnecessary, and any lawbreakers in the Upper and Middle City were banished to the Undercity. Here, they were forced to walk on the original surface of the planet that never experienced sunlight due to the shadows of Taris' enormous skyscrapers, and they faced a lifetime of horrors, characterized by starvation, rakghoul attacks, and rakghoul infections. Even their descendants were condemned to a life of misery. Over the course of generations, the Outcasts increasingly doubted the veracity of the tales of the Promised Land, dismissing them as mere fables intended to prevent the Outcasts from succumbing to despair.
In the year 4056 BBY, non-Humans were also subjected to banishment and compelled to reside in the slums of the Lower City. For a period of ninety years, spanning from 4056 BBY to 3966 BBY, the corrupt government's law enforcement enforced strict socioeconomic divisions upon the inhabitants of Taris. During this period, Rukil's grandfather embarked on a quest to uncover clues regarding the Promised Land, but he was ultimately killed by rakghouls in the Sewers. Rukil's father followed in his footsteps and discovered additional clues pertaining to its location, but he suffered the same tragic fate.
In 3966 BBY, the Republic eventually dismantled the corrupt Tarisian government, bringing an end to the Outcasts' banishment. However, Mandalorian assaults on Taris hindered further endeavors to liberate the Outcasts. In 3964 BBY, the Republic reluctantly relinquished the planet to the Mandalorians, and the Outcasts were once again relegated to a life of damnation. Even after the Mandalorians eventually withdrew from the planet in 3960 BBY, the Outcasts' circumstances remained unchanged.
During the Jedi Civil War, in 3956 BBY, the Republic made a return to the planet to complete the mission they had initiated during the Mandalorian Wars. In the Upper City, Republic scientists developed the rakghoul serum, a remedy for the horrific disease that afflicted the Outcasts, which was replicated numerous times. Just as the Republic was poised to reinforce the socioeconomic divisions, the Sith launched an invasion.
During the Sith occupation, only one of Rukil's followers remained loyal to him: his apprentice, Malya. Rukil, who was too advanced in age to continue searching for clues regarding the Promised Land, dispatched Malya to seek out additional information. She succeeded in uncovering more clues, but she was tragically killed by rakghouls on the Undercity surface before she could return to Rukil.
The Sith maintained strict control over the distribution of the rakghoul serum, reserving doses exclusively for themselves and their Undercity patrol. By the time Revan entered the Undercity, the Sith had virtually exhausted their supply of the serum, although some of the Sith troopers who were killed by rakghouls occasionally possessed a sample of the serum on their corpses.
Upon Revan's arrival in the Undercity, he rescued the Outcasts from the rakghouls by providing a vial of rakghoul serum (discovered on the remains of a Sith Trooper) to an Upper City doctor, Zelka Forn. Forn reproduced the serum and administered it to cure the Outcasts of the disease, and Esala, the village healer, received hundreds of vials from Zelka Forn. Revan also encountered Rukil and aided him in locating the Promised Land. Revan discovered journals belonging to Rukil's grandfather, father, and apprentice Malya in and around the city sewers. He then entrusted the tomes to Rukil, who in turn passed them on to Gendar, who utilized them to pinpoint the location of the Promised Land and guided the Outcasts away from their village.
The Outcasts managed to survive the bombardment of Taris while traversing through the Tarisian sewers, where they discovered no other survivors, with the exception of themselves and Gamorreans. Several months following the bombardment, the Outcasts' exodus from their village to the subterranean realm known as the Promised Land was successfully completed. During the Cold War, a group of heroes discovered that the Outcasts had indeed located the Promised Land; however, it differed significantly from the legends, or at least it did after the bombardment.
The buildings, equipment, generators, and potentially droids had been destroyed, compelling the Outcasts to begin anew. Despite the arduous nature of the task, they had endured the bombardment. However, numerous generations had passed since then, each possessing a "Promised One" who would safeguard the memories of the catastrophic apocalypse and oversee the colonists. The anti-rakghoul serum that Revan had discovered proved sufficient to protect the initial generations, but after the rakghouls developed immunity to it, the vials became ineffective, and babies could not be born with immunity. Some rakghouls evolved into Nekghouls. Unlike the rakghouls, the Nekghouls were incapable of spreading infection.
The Outcasts eventually abandoned the Promised Land and recolonized the Undercity, renaming themselves the Tarisians. However, their efforts were futile, as the Tarisians continued to perish on the surface due to starvation, diseases, and rakghoul attacks. The last successfully preserved time recording revealed that the final generation had lost so much knowledge about the world that they were unaware of how or what had brought them there. The remaining members ultimately succumbed to toxic waste, resulting in their extinction, similar to the fate of the rest of the planet's population many years prior. Information about them remained lost until a Jedi recovered their post-apocalyptic history via hologram recordings from both the Promised Land and the Undercity and provided it to a local Republic history keeper in 3643 BBY.
The sole legacy they left behind was a sample of rakghoul serum, which was discovered by pirates in Zelka Forn's second medical facility, situated in the Undercity within the remnants of their village. It was subsequently given to a Jedi, although the Nekghouls were technically the descendants of the Outcasts. The Jedi group then entrusted it to a Republic scientist with the aim of altering and improving the vial from an infected individual to render it effective. The task was accomplished, and the serum was produced in mass quantities for a third time.
In an alternate, non-canon narrative, Revan provides the serum to one of Davik Kang's henchmen in exchange for a substantial sum of money. In another non-canon plot, Revan gave the three Promised Land journals to Igear, who destroyed them so that the Outcasts could never find the Promised Land.
In Star Wars: Knights of the Old Republic, all the Outcasts residing within the village are human, with the exception of those who have been bitten by rakghouls; however, the infected individuals are then quarantined away from the other members of the village.