The Undercity was a mechanical wilderness on the ground level of Taris, beneath the upper, middle and lower levels and above the sewers and the underground city. It was a dark, wild area inhabited by the Outcasts—people whose ancestors were eternally banished from the surface as a form of criminal punishment.
Several centuries before Taris became a galactic hub the Taris Civil Authority began to construct a planet wide city on the surface after expanding out from a catacomb realm below in which would become the Promised Land. Eventually a sewer system was constructed between the two cities to maintain its infrastructure. Later the Lower City was built on top of it then the Middle City on top of that and finally the Upper City was constructed as the final surface of the planet in which the sky and sunlight were blocked from view. Sometime after the trade lanes shifted, in 4156 BBY, the Rakghoul disease had somehow ended up in the Tarisian Undercity. This caused the city to be abandoned and the sewer system was left to disrepair and was eventually overrun by Rakghouls and Gamorrean slavers, but still served as the planet's understructure.
Abandoned by the Galactic Republic, the Tarisian industrialists began to use the power source of their choice, which produced some form of toxic waste. A century later the pollution poisoned and killed most of the oceanic fauna and flora, and so the planet was swept with famine. The Tarisian elite hoarded the remaining food and wealth for themselves, far more than they needed, while the poor were left to starve and die.
The Tarisian poor, disgusted at what the rich had done, rebelled, beginning the Tarisian Civil War. Millions of innocents died and large sections of Taris were destroyed or abandoned. It was during this time that the self sufficient colony was lost to the Tarisians. The rebellion was crushed in the end and the thousands of remaining disarmed rebels were imprisoned. But the jails could not hold them all, so they were instead banished to the Undercity, unable to return to the surface under penalty of death.
Rakghouls and Gamorreans attack members of the Jedi Covenant in the Undercity.
Dirty, dark, and infested with rakghouls, the Outcasts had never left their prison during their unhappy existence, and there was little hope for their future, or that of their children amongst them, with the exception of Rukil. Rukil was an elderly Outcast who fervently studied and believed in the existence of the mythical Promised Land, a self-sufficient colony (built in an unspecified location) which had been filled with provisions and droid attendants but then forgotten by the rest of Taris long ago. If they could discover the location, said Rukil, they could escape their miserable existence and live in comfort for the rest of their days. Nobody believed in these stories, and most villagers only thought they served to give false hope.
The mechanical wilderness around the Undercity village was extremely dangerous because of rakghouls, semi-humanoid beasts that existed apparently only to attack nearby Humans and infect them with the rakghoul disease. The Outcasts had to keep infected people isolated from the rest of the society because they would inevitably mutate into rakghouls. Dangerous sewers located in the Underlevels were infested with rakghouls and Gamorrean slavers, who used the area for a base.
When the Republic lost the planet to the Mandalorians the Taris Resistance held out in the Lower City for a moment but were later defeated and retreated to the Undercity so they could escape aboard The Last Resort that was based there near the Outcast village.
When Revan first came to the Undercity, he was first greeted by Shaleena, a member of the outcasts who lived in that village. Shaleena told Revan to find and talk to the leader of the village Gendar and if he wanted to, to talk to Rukil. Shaleena told Revan of Rukil's stories of the promised land. Revan first visited Gandar, then immediately went to see Rukil.
At Rukil's behest, the amnesiac Revan sought out the journals of Rukil's apprentice Malya, Rukil's father, and Rukil's grandfather and brought them to the old man while he was trapped on Taris during the Jedi Civil War. Rukil had reached the century mark by then, having survived in the Undercity for a hundred years. Once presented with Revan's findings, he pieced together the journals' clues, finally discovering the location of the Promised Land. He shared the information with Gendar, the village leader, who stated that although the journey would be long and perilous, involving the crossing of rakghoul-infested territory, supplies were high. Faced with choosing to continue their dreary, hopeless lives or pursuing the light of hope that the journals presented, the outcasts departed for the Promised Land.
As a final act towards the Outcasts, Revan was also able to save the Outcasts from the rakghouls by giving a rakghoul serum vial (which he found on the remains of a Sith trooper) to an Upper City doctor, Zelka Forn. Forn was able to reproduce the serum, and used it to cure the Outcasts of the disease. He also used it to cure another Republic soldier who was subsequently killed by rakghouls. The trek proved successful but the building's, generators, equipment and possibly the droids were in ruins.
After several generations, the Outcasts were dying in the Promised Land of Rakghoul attacks, starvation and diseases, the Promised One led them upwards into the new sky filled Undercity and renamed themselves the Tarisians. This however was in vain as the Rakghoul serum became useless with the evolution of the Rakghouls and no one knew how to improve it. There was even a sentient evolution of the Rakghouls into the Nekghouls but unlike the Rakghouls, the Nekghous couldn't spread infection. The last successful recording was made just days before the last generation died out from toxic waste, rendering them extinct like the rest of the population during the bombardment.
During the Cold War that followed the Great Galactic War, the Republic made reconstruction efforts to rebuild Taris, in hopes of defeating the Old Sith Empire. A Jedi managed to recover the lost history in both the Undercity and in the Promised Land and gave it to a local Republic history keeper. He also was able to obtain a Rakghoul serum vial from Pirates, whom they found in Zelka Forn's long abandoned second medical facility where the Outcast village once existed. The serum was improved and mass produced for a third time. Governor Saresh struggled to stamp out corruption in the military and civilian forces across Taris. Thanks to her efforts and her dedication to the cause, the initiative was making more progress than it did in the last decade. However, thanks to the assistance of various members of the Empire, Gravus succeeded in halting the project, and Taris for an unknown period of time, was consequently occupied by Imperial Forces, the Undercity in particular. In 3 ABY, much of the Undercity had been rebuilt by Human colonists, but never again was it a prison for banished criminals or overrun by rakghouls.
If the player chooses to follow the dark side, Revan can sell the journals to Igear, a shopkeeper in the Undercity who destroys the journals out of a jealous desire to gain prestige and profit for himself through the settlers' reliance on his store. This condemns the Outcasts to remain in their village in the Undercity.
Furthermore, the player has the choice to follow Gurney's advice and give the rakghoul serum to Zax at the bounty office in Javyar's Cantina. Should the player do this, the infected Outcasts would be condemned to mutate into rakghouls, and the player will be unable to purchase rakghoul serum from Upper City doctor, Zelka Forn at an inexpensive price.
It is unknown if it was the same Undercity in the Old Republic era or it was a rebuild in the last days of the Rise of the Empire era (in 1 BBY). However in Star Wars: Empire at War the land battle was on only one level and looked quite new. It is possible that what ever was left of it after the bombardment was demolished and rebuilt by the Human colonists.
- Star Wars Knights of the Old Republic: Prima's Official Strategy Guide
- Star Wars: Knights of the Old Republic Handbook
- Knights of the Old Republic Campaign Guide
- Star Wars: The Old Republic Explorer's Guide