The Hundred-Year Darkness was the last of the Great Schisms resulting from a fractured Jedi Order. Just prior to the war, a group of Jedi discovered that the Force could be used to manipulate and distort life itself. By exploring this previously undiscovered knowledge, this group of Jedi fell to the Dark Side as they began to experiment with Force-mutated creatures. When challenged, a rift formed between these Darksiders and those loyal to the Jedi Order. Battle lines were drawn and war soon erupted.
The Dark Jedi used their newly discovered knowledge to assemble an army of colossal leviathans imbued with the power of the Force. Some of these monstrous beasts were even mutated with the power to absorb the life-essence of soldiers on the battlefield. They were stored within the leviathans in a state of chaos and torment, and only when these creatures were destroyed were the souls stored within able to be set free.
The war raged for a century. Finally, the Jedi were able to push back the Dark Jedi forces to the remote planet of Corbos. There the two factions devastated the landscape as the war raged across its surface with hoards of soldiers and leviathans lumbering across the battlefields to meet forces loyal to the Jedi. In the end, the Jedi were able to drive the Darksiders not only from Corbos, but from the Republic itself. Setting out for an obscure and little-known region of space, the Dark Jedi encountered a primitive species known as the Sith.
These Sith worshipped the Dark Jedi as gods who eagerly took up that mantle to become the Dark Lords of the Sith. Over the millennia, the Jedi and the Republic forgot these outcasts and left them free to rule in their unknown region of space. Soon the Sith lost all records of the Republic and left undisturbed built up the Sith Empire to include hundreds of worlds. Their knowledge of the Force was passed down through the generations and came to be known as Sith magic which included many of the secrets the original Dark Jedi brought with them - alchemy, illusions, and mutations. It was not until a pair of explorers rediscovered the Sith Empire by chance that the Sith found their way back to the Republic and once again proved to be the bane of the Jedi.