Overview:

The Great Sith War, also known as the Sith War, the First Sith War, the Exar Kun War and the Great Shadow Crusade to the Mandalorians, was a galaxy-wide conflict which took place in 3996 BBY. The first conflict of the Old Sith Wars, the Great Sith War saw the rise of the Sith, once again in opposition to the Jedi Order. The roots of the conflict are traced back to the fall of Freedon Nadd from the Jedi way and his ascension to Dark Lord of the Sith. Eventually his dark influence was discovered on the planet Onderon and the ensuing battle saw the Jedi temporarily break Nadd's hold on the remote planet. This fight against the resurgent power of the Sith was a warning to the Jedi that something ominous was brewing.

The influence of the Sith, however, again resurfaced during the Freedon Nadd Uprising, which managed to establish the means for the Krath to grow in power and influence. Using Sith knowledge that they had gained from Nadd and his followers on Onderon, the Krath instituted a bloody coup in the Empress Teta system and began a series of campaigns aimed at Republic space. After several defeats at the hands of the Krath, Jedi Knight Ulic Qel-Droma advised the Jedi Order that he would infiltrate the Krath and destroy them from within. During his undercover mission, Ulic fell to the dark side and joined forces with another fallen Jedi-turned-Sith by the name of Exar Kun. Eventually, Kun was anointed the new Dark Lord of the Sith with Qel-Droma his apprentice, and together they ushered the galaxy into a new Sith age. 

With their allies, Exar Kun and Ulic Qel-Droma launched a series of deadly campaigns against the Republic, eventually making their way to the heart of the galaxy—Coruscant. Following this battle Kun and Qel-Droma launched an assault on Ossus, in an attempt to raid the planet of its Jedi artifacts. However, the battle of Ossus saw the defeat of the Sith forces, the capture of Ulic Qel-Droma, and the retreat of Kun to his base on Yavin 4. Broken and humbled, Qel-Droma agreed to lead the combined forces of the Republic Rim Fleet and the Jedi to Yavin 4, in an effort to defeat Exar Kun. Under the combined efforts of the Jedi, Kun was defeated and the Great Sith War brought to an end.