The Outcasts were a group of exiled civilians forced to live, work and survive below the surface of the planetary cities of Taris. A leader of one such Outcast group known as Rukil made contact with Revan and his party shortly before the Bombardment of Taris by Darth Malak.

Rukil revealed to Revan that there was a sacred location, prophesized as the ‘Promised Land’. A location left untouched by the surface-dwelling Tarisians and untainted by the presence of the Rakghoul Virus. A safe haven for those living in the poverty of the Undercity that were fighting to survive. It is believed during this time that Revan not only helped Rukil and the Outcasts by clearing out nearby rakghouls, allowing Rukil and his group to make the journey, but also that Revan gave Rukil’s Outcasts a Rakghoul serum that could be used to fight off the infection.

However the success of the Outcasts making the ‘week long’ journey of the Undercity would come under jeopardy as Revan fled the planet when Malak began his orbital bombardment of Taris. As the cities of Taris collapsed under themselves, the fate of Rukil and the Outcasts faded into oblivion.

During the Cold War era hundreds of years later, a Republic Historian by the name of Hacken Berge located on Taris for the restoration efforts found claims of buried holorecordings from the Outcasts and their leaders. Hacken Berge connected the dots between Revan’s history with the Outcasts and asked Republic Heroes (Player characters) for help in uncovering the mystery.
These Republic Heroes would scour the now unearthed surfaces of Taris and uncover a series of holorecordings belonging to the Outcasts that spanned decades or even a century. However the fate of the Outcasts on Taris would become a depressing reality.

Several holorecordings left by Rukil, and his successors; Ku-ki and Lurr, would reveal that when they first arrived, the Bombardment of Taris had left all the droids, computers and technology defunct or destroyed. This meant that they would have to rebuild the Promised Land themselves.
As time went on and generations passed, the leader of the Outcasts would promote a new “Promised One” that would spread the history and teachings to future generations. Unfortunately as time marched on, the supply and effectiveness of the Rakghoul Serum provided by Revan dwindled before becoming useless after only several generations. This meant that they were once again succeptible to the Rakghoul Plague, and the new Nekghouls that inhabited Taris.

Several more generations would go by until the remaining Outcasts left the Promised Land, it was during this time that the history of their plight had finally faded from existance as they renamed themselves ‘Tarisians’ and resettled on the exposed surface. However it would not be for long until all of them died from exposure, rakghouls or toxic waste, as the original Tarisians had many decades ago, during the Bombardment.

Sources:

Rukil and Igear, Taris – Knights of the Old Republic
Taris Republic SideQuest: Chasing History – Star Wars: The Old Republic