The Fourth Great Schism represents a significant division within the Jedi Order's structure, occurring around 2000 BBY.
At approximately this time, Phanius, a Jedi Master, forsook the Jedi principles in search of different doctrines, which guided him to resurrect the dark-side devoted Sith Order. It was widely accepted that the Sith had been eliminated for hundreds of years. However, the corrupted Jedi Master disproved this notion by locating several surviving Sith cults scattered across the galaxy. Taking control of these cults, Phanius proclaimed himself a Dark Lord of the Sith, adopting the name Darth Ruin.
Fueled by the Dark Lord's rise to power, fifty more Jedi emulated Ruin's actions, abandoning the Jedi Order to join his expanding Sith empire. These occurrences led to the emergence of the New Sith, thereby starting a millennial conflict—the New Sith Wars—against both the Galactic Republic and the Jedi.