The Third Great Schism was a conflict between different factions of the Jedi Order in the centuries prior to the Great Sith War. The schism was a result of a group of Jedi following the dark side of the Force. These Dark Jedi were later destroyed when they attempted to use the ancient Cosmic Turbine in the Vultar system, ending the Third Great Schism.
In the centuries after the death of Freedon Nadd, the Jedi Order came to the realization that there would always be Jedi who struggled with the dark side of the Force. As time went on, a group of Jedi began to practice the dark side and they became Dark Jedi. The Jedi and Dark Jedi fought a civil war on Coruscant, but the Dark Jedi were driven away from the planet and found refuge in the Vultar system.
The exploration of the Vultar system led to the discovery of ancient machines that belonged to the long-dead Celestials. These machines led the Dark Jedi to conclude that the entire system had been artificially constructed, rather than developing naturally. Recognizing the power of these machines, the Dark Jedi planned to use them, including the machine they referred to as the Cosmic Turbine, against the Jedi Order. However, the Dark Jedi did not know how to control the machines. After attempting to utilize them, an accident occurred that destroyed the entire Vultar system, and the Dark Jedi along with it. This incident brought an end to the Third Great Schism.
The Jedi Order's belief that it would continue to face its members falling to the dark side was again proven true in the centuries after the Third Great Schism. During the Great Sith War, a new Sith sect arose, led by fallen Jedi such as Ulic Qel-Droma. The Jedi would continue to face threats from the Sith for millennia to come.
- Star Wars: The Old Republic (First mentioned)(Mentioned in Codex)