First Great Schism

The First Great Schism was one of the first splits between followers of the Jedi Order and those who refused to follow the Jedi hierarchy's dictates. To its contemporaries it was a conflict without name, and known as the Great Schism by historians that followed, until the Second Great Schism necessitated referring to this conflict as the First.

Origins
For over ten millennia, a society of Force-wielders had lived in relative peace on the Deep Core world of Tython, when they decided to relocate in order to step outside the growing sphere of influence of the fledgling Galactic Republic. They had made their new home on Ossus in the Outer Rim.

Already at this point, the Order had grown doctrinaire in its practices and, to some, too rigid to allow growth. A young Jedi named Arden Lyn was one such believer, and found a leader in fellow disgruntled Order member Xendor. Xendor, a Kashi Mer, had grown vocal in his criticism of what he perceived as the Order's increasingly exclusionary ways.

Xendor initially petitioned the leaders of the Order to depart Ossus in order to create a new academy. There, he hoped to teach and study alternative traditions of the Force, such as Dai Bendu, the Palawa, the Way of the Dark and the Protectorate of the Hidden, to name just a few. The leaders of the Order refused Xendor's request, but he exited in defiance despite their decision.

A new academy
Xendor and Lyn departed Ossus to establish their new academy on Lettow. They would not be the only ones to defy the dictates of the Order; larger and larger groups left Ossus to join Xendor's new group and study mysteries of the Force that had been forbidden to them under the Order. Eventually, Xendor's followers would come to be known as the Legions of Lettow.

The newcomers to Xendor's academy pledged their allegiance to him and to the protection of their new order. They swore to defend their right to pursue knowledge of the Force without the oversight of the Jedi hierarchs, deeming them unfit to dictate which avenues were correct and which were forbidden.

The Legions of Lettow initially wished to be left alone by the Jedi, but the peace would not last. The Jedi Order branded the Legions as dissenters, and both sides began to arm themselves for the conflict that had become inevitable.

Open war
The truth of which side began the war is lost to history, but one of the first battles was fought on the Jedi capital world Ossus. Claiming to wish to end the conflict early and prevent it from spilling into the Republic, Xendor led the Legions in an attack on Ossus. Their attack failed, and the war continued unabated.

Lyn claimed, millennia later, that Xendor tried to alert the young Republic to the threat he believed the Jedi posed to them; that they would guise themselves as protectors and guardians while they secretly schemed for power. In this, too, Xendor failed, and the Legions failed to gain any traction in the Republic against their Jedi enemies.

The conflict spread across a number of worlds as it intensified. War spread to Chandrila, Brentaal, the Republic capital Coruscant, and Metellos as well, before a significant battle on Columus. At Columus Xendor engaged the Jedi Awdrysta Pina, known by his nickname Green Blade, and was slain in single combat.

Lyn would later muse that both she and Xendor had admired Pina as a free-thinker in his own right; one who preferred to travel to unknown places in search of mysteries of the Force unknown to the Jedi Order. Ultimately, however, Pina believed that the order and structure that the Jedi represented were the foundations of civilization itself and so he stood with them rather than with Xendor.

The Jedi forces commanded by Pina held a unique advantage over the Legions which even Lyn was forced to admit: their dedication to order allowed them to subsume their individuality into a larger collective and fight as one–not unlike the Force ability known as battle meditation. Xendor's followers refused to embrace any such tactic, believing it insectile and an affront to individuality.

Extermination of the Legions
Victory over Xendor, however, was not sufficient for the Jedi to declare victory in the war. Instead, forces under Pina were sent to Lettow to attempt to exterminate the last of Xendor's followers. A fierce battle was fought, allowing just enough time for the last survivors of the Legions to flee beyond the boundaries of Republic space.

Lyn was eventually cornered by Pina on Irkalla, after fleeing into regions of space unknown to all but Xendor. The two engaged in a duel in which she was disarmed. Unwilling to concede defeat, she channeled her strongest emotions–rage and sorrow, love for Xendor, and despair over what she believed had been unfairly taken from them–through the Kashi Mer talisman, destroying Pina's sword and mortally wounding him in the process.

In his dying breaths, he drew on his own unique Force talent, Morichro, to defeat Lyn. The spell, in essence, placed her into a deep stasis. She would not be resuscitated until the era of the Great Jedi Purge, over 24,000 years later.

Behind the scenes
For many years, it was believed that the First Great Schism was the event that resulted in the formation of the Sith Order, with Naga Sadow's entry in The New Essential Guide to Characters somewhat confirming that, saying that Sadow was the first Sith Lord in 20,000 years to reestablish contact with the Republic. However, Lucasfilm had intended the Hundred-Year Darkness as the event which lead to the establishment of the Sith Order, and was retconned as such in The New Essential Chronology. Dan Wallace, who worked on both projects, has since stated that the Naga Sadow entry was an error.