Fallen Jedi, also known as dark Jedi, rogue Jedi, and renegade Jedi, were Force-sensitives, frequently former Jedi, who chose to forsake some or all of the tenets of the Jedi Creed and embrace the dark side. Although "fallen Jedi" originally referred to a Jedi who had fallen to the dark side, it could also refer to uninitiated Force-sensitives who received no Jedi training but began their careers under another fallen Jedi. The first fallen Jedi was believed to be Xendor, who was the first member of the Order of Jedi Knights to embrace the darker aspect of the Force.
Fallen Jedi also became a term of note during the Great Sith War and the Jedi Civil War, where such Jedi served under the banners of Exar Kun and the duo of Revan and Malak, respectively. Thousands of years later, dark side adepts such as Palpatine would resurrect the term "dark Jedi", with agents serving under him bearing the title. Those whom High Inquisitor Jerec controlled came to be known as the Seven Dark Jedi.
Description and comparison to the Sith[]
Despite the fact that both groups called upon the more aggressive powers of the Force, fallen Jedi were not the same as the Sith.

A dark Jedi warrior.
The term fallen Jedi was a collective name of dark side practitioners (be they former or rogue Jedi) that used Force powers and a lightsaber, but were not always members of a certain organization; whereas the term Sith referred to a definite heritage or ideology. Some fallen Jedi were allied to the Sith or other organizations, while others were loners. Because they were notoriously treacherous, fallen Jedi seldom formed well-organized groups on their own in order to expand their following or reinforce their strength; one notable exception to this was the cult of Bpfasshi Dark Jedi that terrorized Republican and Separatist space during the Second Clone War. Fallen Jedi could also use their power for personal gain, becoming mercenaries or assassins.
The Sith, however, were various orders which held to their own unique interpretations of the Force, held to their own distinct theologies and cosmologies, followed their own codes of honour and conduct, studied their personal histories, studied their own skills and techniques, and possessed arcane secrets unknown to renegade Jedi.
History of the fallen Jedi[]

Xendor, the first Jedi Knight to explore the forbidden aspects of the Force.
The First Great Schism and the Hundred-Year Darkness[]
After the Order of Jedi Knights was established, divisions began to appear within the order's structure, leading to the First Great Schism, (c. 24,500 BBY), when several Jedi were discovered delving within the "dark" side of the Force. The renegade Jedi Xendor and his followers allied with the mistress of Teräs Käsi Arden Lyn and her Followers of Palawa from Bunduki, establishing the Legions of Lettow. After religious tensions with the Jedi blossomed and escalated into all-out violence, a war broke out between the Jedi Knights and Lettow Legionnaires that lasted a hundred years. The Lettow were finally defeated at the Battle of Corbos and exiled into the far reaches of space.
These exiles finally landed on the Sith planet Korriban, deposed the Kissai governor reigning there, and liberated the population from the tyranny of the Sith Empire. The legionnaires were eventually annihilated, wiped out in a planet-sterilizing bombardment orchestrated by the reigning Kissai overlords of the Sith Empire. The deaths of the Lettow inspired the downtrodden Sith on other worlds in the Empire to rise up against their Kissai masters, initiating the Sith Civil War which led to the rise of the first Sith Lords.
The Second Great Schism and the Great Sith War[]

Exar Kun, the first Dark Lord of the Sith and founder of the Sith Brotherhood.
Exar Kun was a talented individual, strong in the ways of the Force, who began his Jedi training under Master Vodo-Siosk Baas on the planet of Dantooine. Despite his master's teaching, Kun was an overly ambitious and curious student who secretly studied any available knowledge pertaining to the Golden Age of the Sith. In 4000 BBY, believing that the Jedi teachings had somehow lost their direction over time, Kun investigated clues from his studies of the Sith and followed these clues to the Sith legacy on Onderon.
Enlisting the help of a pair of Naddists from that planet, Kun traveled to the moon of Dxun intent on discovering any Sith secrets that laid buried in the tomb of Freedon Nadd. Encountering the heavy Mandalorian iron that Master Arca Jeth had had Nadd's tomb forged from, Kun used his lightsaber to gain entry to the sarcophagus, where he found the skeletal remains of the long-dead Sith Lord, which were still clothed in Nadd's black armour. During his search of the tomb, Kun was stopped by the spirit of Nadd, who warned the young Jedi that the dark side was dangerous, but rewarding. Before Kun could reply, Nadd revealed a series of scrolls written by Naga Sadow, which had been hidden in a compartment under his remains, and advised Kun that a great destiny awaited him in the power of the dark side. Using the scrolls that Nadd had given him, Kun continued his exploration and search for Sith knowledge.
Following the words contained in the scrolls, Kun arrived on the ancient Sith world of Korriban to continue his search. Making his way to the Valley of the Sleeping Kings, Kun investigated a series of large crypts which contained the remains of ancient Sith Lords. However, in a test from Nadd, Kun was nearly killed when a tomb he was investigating was caused to collapse around him. As Kun cried out in pain and for help, Nadd appeared and told him that he would save him if Kun surrendered himself completely to the dark side. Kun agreed by making a hollow promise, but he was already too far down the dark path to make it back to the light on his own accord. The power of the dark side threw the collapsed stone off of him and filled Kun, healing his broken body, and left him laying naked on the sun-baked earth of Korriban.
Still believing that he could learn from the Sith and not be tempted by their ways, Kun continued his exploration of the dark side. His fall, however, had already begun and several Jedi masters across the Republic could sense the currents of the Force change as Kun's choices began to influence interstellar events, shifting them into something unspeakably dark and dangerous.
Following Kun's excursion to the Sith world of Korriban, Freedon Nadd directed the corrupted Jedi to the jungle moon of Yavin IV, where he would complete his training in the ways of the Sith.
Arriving on the forested world, Kun was confronted by the Massassi, mutated descendants of the warriors who had served the Lord of the Sith Naga Sadow after his banishment from the Sith Empire and exile to the jungle moon. Due to the episode in the tomb on Korriban, Kun had since been unable to use the power of the light side and refused to entirely accept power provided to him by the dark side; because of this, Kun was easily captured by the Massassi and taken back to their main temple. Under the direction of the Massassi priest Zythmnr, Kun was to be sacrificed to an ancient Sith creation of Sadow's. As Kun was on the verge of death in the grips of the giant creature, Freedon Nadd's spirit appeared to Kun and told him that to save himself, he must concentrate on his anger and use it to free himself. Using the dark side to grab a Sith amulet from Zythmnr, Kun focused his anger through it and used it as a weapon to free himself through blasts of energy. Finally comprehending the power that Nadd had been trying to teach him, Kun fully embraced the dark side.
Kun proceeded to kill the enormous beast that had tried to kill him, and slaughtered his captors without hesitation. Nadd was pleased while he watched Kun give into his new-found strength, and believing that Kun was ready, ordered him to use his new Sith powers to create Nadd a new body. Kun, however, had grown agitated and tired with what he perceived as constant interference from Nadd, and as a result used his powers to destroy Freedon Nadd's spirit — removing its taint from the Galaxy forever. With Nadd gone, Kun crowned himself the Dark Lord of the Sith, the arch-magus of a new Sith religion which would build on the best teachings of the old Sith and the Jedi Order, superseding them both. Enslaving the Massassi, Kun forced them to construct new temples and repair the relics left from the time of Naga Sadow, transforming Yavin IV into his own personal fortress. Kun planned to use these temples and buildings for Sith ceremonies and rites which would help focuses his Sith magic, thus making it more powerful.
As his power grew, so did Kun's ambitions. For Exar Kun to usher in a new golden age for the Sith, he believed that he must first have a strong and united group of fellow Force users who followed his ways. To this end, Kun travelled to the Jedi library world of Ossus and attempted to persuade a large number of Jedi to leave the Order and make him their new master. Arguing that the Jedi masters had deliberately kept information from them to make them weak, Kun gave an impassioned speech to those gathered, managing to sow seeds of doubt in the Jedi Knights listening. Following the Great Android Revolution, which saw the death of many Jedi Knights, some Jedi had become disillusioned with the Order and had begun to question their own abilities in the Force. After hearing how Kun had been able to learn many secrets from the spirit of Freedon Nadd, and witnessing demonstrations of Kun's new powers, several Jedi Knights made the decision to follow Exar Kun to Yavin IV, where he promised to teach them that which the Jedi Order had neglected to.
While his converts prepared for their journey to Yavin IV, Kun went about his second objective which had brought him to Ossus. In his quest for dark side knowledge, Kun was eager to acquire the famed Sith holocron of Odan-Urr's which the Jedi master had discovered a thousand years earlier during the Great Hyperspace War. Forcing his way into the private chambers of Odan-Urr, Kun confronted and killed the old Jedi and took possession of the Sith holocron. Kun, however, was confronted by his converts while in the act of removing the artifact. Sensing an opportunity to add credit to his lies, Kun told the knights that Master Odan-Urr had quite suddenly passed away. However, he said, before Odan-Urr had died, he had declared Kun to be a Jedi master and had decided to pass the Sith holocron into the care of Exar as a symbol of his promotion. With the respect he needed now firmly cemented, Kun boarded his ship, Starstorm One, along with his converts and departed Ossus for Yavin IV.
Meanwhile, another Jedi, Ulic Qel-Droma, although motivated by good intentions, also began his journey to the dark side. He had became Emperor of the Empress Teta system and leader of the Krath, a Sith-inspired cult that had formed after the Freedon Nadd Uprising and had learned much of their power from Sith artifacts that survived the conflict. Even before Qel-Droma joined their ranks, the power-hungry and very hostile Krath had staged a violent military coup in their native system, crushed all resistance and declared war against the Galactic Republic, carrying out also brutal attacks on the Jedi Order which resulted in massive Jedi casualties.
Learning of the Krath in 3997 BBY, Exar Kun set off to their headquarters with intent to eliminate the self-proclaimed followers of Sith teachings. However, during the lightsaber duel between Kun and Qel-Droma, the spirits of a number of long-dead Sith Lords appeared, including that of Marka Ragnos, and they put a stop to the duel. Ragnos named Kun the master of the reborn Sith and Qel-Droma his apprentice, believing that they would restore the Golden Age of the Sith. Uniting their forces, both Sith Lords vowed to raze the Known Regions and conquer both the Republic and Jedi. This led to the rebirth of the Sith Empire. Empress Teta, a very heavy-fortified planet in the Deep Core and the capital of the Krath faction, became the second Sith stronghold.
The Great Sith War, which tore the Republic apart and led to massive casualties, lasted ten years; only after Qel-Droma surrendered and gave strategic secrets up to the enemy did the war draw to a close in 3986 BBY. Within months of Qel-Droma's defection, the Krath forces and their Mandalorian allies were routed and the Republic and the Jedi staged an invasion of the Yavin system. Exar Kun and most of his followers died on Yavin IV in a massive conflagration which consumed nearly the entire moon, bringing an end to the Brotherhood of the Sith and their Sith Empire.
The Third Great Schism and the Jedi Civil War[]

Revan and Malak, commanders-in-chief of the Jedi Empire.
Towards the end of the Mandalorian Wars in 3964 BBY, the Republic's Jedi commander Revan visited the ancient Trayus Academy on the Lettow world of Malachor V. There, he fell under the influence of the dark side. While unearthing the planet's buried secrets, Revan found historical traces of the forgotten Star Forge, a treasure of the Rakatan Infinite Empire.
Not long after, Revan forced a final battle with the Mandalorians above Malachor V. After ending the Mandalorian Wars with a crushing victory, Revan and his friend Malak led the remainder of their forces (a third of the Republic's fleet) into the Unknown Regions under the pretense of tracking down the remaining Mandalorians, but they did not come back and sent no word of their whereabouts. In time, the Republic feared them lost in some tragic disaster.
From the deck of the Star Forge, hidden in the unmapped Rakata system, Revan founded a new empire. Adapting the teachings of the ancient Lettow to his own needs, he took his friend Malak as his apprentice and christened himself Dread Lord of the Jedi. A year after their apparent disappearance, Revan and Malak returned at the head of a massive invasion fleet made up of Republic vessels as well as strange hybrid technologies built by the Star Forge. They were crewed by experienced ex-Republic veterans and commanded by some of the finest officers that the Republic had produced. Calling themselves the Jedi Empire, they declared war on the Republic.
With Revan at the helm, the Imperial invasion was all but unstoppable. His rogue Jedi assassins secretly flooded into the Republic from their base on Malachor V, killing and/or abducting Jedi throughout Republic space. On Imperial outposts on Rakata Prime and Dxun, Revan's most faithful servants broke captured Jedi and led them to the dark side. The Imperials also conspired with Czerka Corporation, one of the richest and most powerful Republic-spanning companies of the time, to amass much needed resources.
As the Imperial conquest continued unchecked, worlds began to secede to the Imperial invasion fleet voluntarily, either out of fear or because they believed the corrupt and ineffectual Republic was finally about to fall. The Republic, however, still had one last card to play — the Jedi. Time and again, the Republic armadas were saved from certain destruction by the prowess of a young Jedi Knight named Bastila Shan. During a long string of Republic losses, her skills in battle meditation staved off utter defeat.
When the Jedi Empire was on the brink of victory, Revan's flagship was invaded by a Jedi strike team during an ambush. Led by Shan, the Jedi were able to corner Revan on the bridge. Ever ambitious, Malak fired upon his master's vessel, hoping to destroy all of his enemies with one swift stroke, but all did not go as he had planned. The ship was destroyed, but the Jedi were able to escape with a captive Revan.
After the apparent death of Revan, Malak took his master's title and continued his former master's war of conquest. Malak's primary objective quickly became the assassination or capture of Bastila Shan. He got his wish when his fleet overwhelmed and boarded her flagship, the Endar Spire, during an Imperial ambush over the world of Taris. She was captured by Malak and after being tortured, fell to the dark side and became his new apprentice.
Having discovered the ancient Rakatan Star Maps on Dantooine, Tatooine, Kashyyyk, and Manaan, an amnesiac Revan and his crew were able to piece together the location of the Star Forge. The Republic armadas were summoned to Rakata Prime and a massive space battle took place between the Republic and Imperial forces. After a confrontation with Revan, Shan returned to the light side and betrayed her master by using her battle meditation to aid the Republic forces. The Jedi Civil War ended as Malak died at the hands of his old master. With Malak gone and the Star Forge destroyed, the infrastructure of the Jedi Empire collapsed. The remaining dark Jedi succumbed to infighting and virtually wiped themselves out.
The Kunist Insurrection and the Pan-Republic War[]

A pair of Kunists, descendants of the Sith sleeper agents who infiltrated the Order of the Jedi Knights in the closing years of the Great Sith War.