Once we were Jedi of the Galactic Republic… brothers in the Force. But a schism between darkness and light turned Jedi against Jedi. Our ancestors were defeated, driven off… cast out…
—Marka Ragnos
The Second Great Schism, also known as Hundred-Year Darkness, and The Great Schism, was a conflict that began in 6,968 BrS, and continued until 6,865 BrS. The war, one of the longest in galactic history, originated with a group of Dark Jedi dissidents, who created monstrous armies to battle the Jedi Order and the Galactic Republic.
After a century of conflict, the Dark Jedi lost the war in it's final battle at Corbos, but they went on to conquer Korriban, and found the Sith, the greatest threat to the free beings in the history of the galaxy.