Korriban was the ancient homeworld of the Pure-blood Sith race, as well as the Sith Empire. The Sith Empire was founded by a group fallen Jedi known as Exiles, who fled to the system after being defeated by the Jedi order. Using their powers of the Force to made themselves as gods of the Sith race. Year's of inbreeding with the exiles and the Sith, made the term Sith for both the Sith race and their fallen Jedi masters.
Korriban was discovered by the Galactic Republic by 2 hyperspace explorers Gav and Jori Daragon, the siblings were captured by the Sith who branded them as spies. One of the Sith lords Ludo Kressh ordered the two be executed, but the second Sith lord Naga Sadow wanted to use them to rise to power. Naga Sadow orchestrated the escape of two explores , he let Jori escape to the Republic and kept Gav as his apprentice. After eliminating Kressh, Naga Sadow united the Sith under his rule, and crowned himself as the Dark Lord of the Sith. With his new Empire Sadow invaded the Republic in the Great Hyperspace War.
When Naga Sadow retreated back to Korriban to regroup his forces, however he was ambushed by Ludo Kressh who faked his death. While to two forces were fighting each other the Republic armada arrived and annihilated both forces. After the war Korriban was slowly forgotten again.
Korriban was guarded by a small Republic force, it wasn't until the Sith Empire sent a strike force to retake the planet for the Sith.