The Endor Sith breakaway was an event affecting the Night Jedi of Endor. It occured when the Sith Incursionus persuaded the Night Jedi Blademaster Diaeta to completely turn to the dark side and join the Sith. Together, they would form a Sith Citadel on Endor, which would be later attacked and captured by the Night Jedi.
In 38 ABY, the Sith Incursionus made his way to Endor, and encountered the Night Jedi. After a short series of skirmishes, he was captured and imprisoned by the Night Jedi.
However, during his imprisonment, he exerted pressure on the blademaster Diaeta, a former Sith, to return to his Sith ways. After some time, he succeeded, and with the help of Diaeta, escaped from the Night Jedi Citadel. Over the next few weeks, they hid in the forests of Endor, before setting up a Sith Citadel near the ruins of Hanging Moss Village.
One of the Night Jedi, Tanek Ara, was recruited to infiltrate the new Sith faction, and report back on their position. He made contact with Diaeta, and pretended to join the Sith too. This enabled him to report to the other Night Jedi the position of the Sith Hanging Moss Citadel.
The Night Jedi then planned an attack against the Sith Citadel. Most of the high ranking Night Jedi on Endor were involved, along with a contigent of the Night Jedi defence forces. The Night Jedi broke into the Sith Citadel, and engaged in combat with the Sith for a short time. Tanek revealed his true allegiance, and the Night Jedi forces stormed and captured the Citadel. Diaeta was recaptured, and taken back to the Night Jedi Citadel by Pryce Fisto However, Incursionus managed to escape, and remained on Endor making hit-and-run attacks against the Night Jedi until he was finally confronted by Jobie Wan Kenobi. After a long fight, Incursionus was eventually captured and re-imprisoned.
This event served the purpose of demonstrating to the Night Jedi the need to keep their defensive forces continually ready to monitor activity on Endor. They would later use the Hanging Moss Citadel and surrounding area as part of a military base to cover that portion of the moon.