Dark Jedi, also known as "Fallen" Jedi were Force-sensitives, frequently former Jedi, who chose to deny the light side of the Force and/or follow the dark side. Although "Dark 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 Dark Jedi. Others were simply dark-side users who did not follow the teachings of the Sith or other dark side organizations. In some cases, Dark Jedi also included "artificial" Force-sensitives who served the dark side, such as the Reborn and the Shadowtroopers. The first Dark Jedi was believed to be Xendor, who was the first member of the Jedi Order to fall. Other forerunners included Ajunta Pall and others who, after the Hundred Year Darkness, were exiled from Galactic Republic space and became the ancient progenitors of the Sith Lords.
Dark Jedi also became a term of note during the Great Sith War and the Jedi Civil War, where fallen Jedi served under the banners of Exar Kun and the duo of Darth Revan and Darth Malak, respectively. Thousands of years later, Sith Lords such as Palpatine would resurrect the term, with agents serving under him bearing the title. Those whom the Miraluka Jerec controlled came to be known as the Seven Dark Jedi.
Despite the fact that both groups called upon the dark side of the Force, Dark Jedi were not the same as the Sith, though the first group of the Dark Jedi were the predecessors of the Dark Lords in the ancient Sith Empire.
The term Dark Jedi was a collective and vague name of dark side practitioners (be they fallen or rogue Jedi) that used Force powers and a lightsaber, but were not always members of a certain organization; whereas the term Sith refers to a definite heritage of cult or ideology. Some Dark Jedi were allied to the Sith or other organizations, while others were loners. Because they were notoriously dangerous, unstable, and treacherous, Dark 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 Miraluka Dark Jedi Jerec and his retinue of minions.
The Sith, however, was an order that followed their code and studied their history, studied their own skills and techniques, and possessed secrets like Sith alchemy and Midi-chlorian manipulation. Furthermore, after the Rule of Two was established, the Order of the Sith Lords was limited to only two members, a master and an apprentice, whereas the Dark Jedi had no organization and no such limitations.
The Sith would often seduce Dark Jedi to work for them, without really granting them any knowledge of Sith techniques. Yet most Dark Jedi were willing to serve the Sith, and saw it as a way to further their knowledge of the Force. Like the Sith, they usually used lightsabers with red blades, but were also known to use a wide variety of saber colors.
Many dark side practitioners, such as Count Dooku, considered the power of a Dark Jedi to be nothing compared to the power of the Sith. Other individuals, like Darth Vader, practiced both Dark Jedi and Sith techniques. According to Dooku, a prime difference between Dark Jedi and Sith is that the Sith "lack fear".
After the Jedi Order 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 Dark Jedi Xendor and his followers allied with the master of Teräs Käsi Arden Lyn and her Followers of Palawa from Bunduki. In a climactic battle, Xendor was slain, Jedi Master Awdrysta Pina stopped Arden's heart with morichro, and Lyn killed Pina with a Kashi Mer talisman. After the war, the rest of these dissidents—known as the Legions of Lettow—were defeated.
For over the next seventeen millennia, crises involving the Dark Jedi were rather isolated. One example involved a female Jedi Knight named Halbret who had brought together a contingent of Jedi Knights around 11,500 BBY to defeat a powerful Dark Jedi who had enslaved the Kathol species.
Around 7,000 BBY, however, a second war broke out between The Jedi and Dark Jedi that lasted a hundred years. The Dark Jedi were finally defeated at the Battle of Corbos and exiled into the far reaches of space. These exiles finally landed on the planet Korriban, enslaved the native Sith, and eventually interbred with them and created a new Force organization that eventually would plague the Galaxy for millennia: the Sith cult.
After their defeat during the earliest Jedi Civil Wars, called the Great Schisms, the adherents of the dark side fled from Republic space and settled on the world of Ziost where they ruled over the barbaric red skinned Sith like gods. In time, the Dark Jedi and the remarkably force sensitive Sith natives interbred through the use of Sith alchemy and became one people. The united Sith formed an immensely rich and powerful empire built upon sorcery and dark side fueled technology quite possibly adapted from the scattered ruins of the earlier Infinite Empire of the Rakata.
Over the centuries, the Dark Jedi exiles were forgotten by the Galactic Republic even as all records of the Republic were lost by the Sith. The two civilizations thrived in isolation, each ignorant of the other.
Many Jedi seduced by Exar Kun became Dark Jedi. After the death of Kun, many Dark Jedi still existed and served under new Sith Lords like Darth Malak and Darth Nihilus. During the era of Lord Kaan, many dark-siders were considered Sith Lords and very few Dark Jedi were ever seen in the following millennia, until the Clone Wars.
Many Jedi joined Revan and Malak when they joined the Mandalorian Wars, and almost all of them became Dark Jedi when the Jedi Civil War started. Jedi who hadn't followed them were captured and turned to Dark Jedi.
2,000 BBY, Umbaran Jedi Master Phanius fell to the dark side. He left the Jedi Order with fifty Jedi Knights and became Darth Ruin. He eventually established a New Sith Empire that later resulted in the New Sith Wars.
After the death of Darth Maul, Darth Sidious apprenticed Darth Tyranus who in turn apprenticed several Dark Jedi, including Asajj Ventress and Sora Bulq. He also supposedly had under his wing the Jedi-turned-Dark Jedi, Quinlan Vos.
27 months after the Battle of Geonosis, the Bpfassh Dark Jedi crisis occurred. A group of Bpfasshi Dark Jedi were responsible for a rampage of terror and destruction throughout the Bpfassh system and, indeed, the entire Sluis Sector. A Jedi task force led by Yoda was sent to stop them and quell the uprising.
Near the end of the Clone Wars, everything went wrong for Dooku. Ventress, after faking her own death, fled to an unknown location. Sora Bulq was killed by Quinlan, after the latter had turned back to the light, during the Siege of Saleucami. Another Dark Jedi named Tol Skorr, was also killed by Quinlan during the battle. A few weeks after the siege, the Count himself was killed by Anakin Skywalker onboard General Grievous's flagship, the Invisible Hand. Soon after, Skywalker became Darth Vader, Palpatine's newest apprentice and the Sith ruled the Galaxy once more. This would lead to the employment of many more Dark Jedi.
During the span of the Galactic Empire, Palpatine established a number of organizations composed by Dark Jedi, including the Inquisitorius, the Emperor's Hands and his personal Dark Side Elite. Many AgriCorps Padawans were captured by Vader after the execution of Order 66. The younglings that Vader did not kill (which were very few) were trained in the ways of the dark side by several Dark Side Adepts, including Vader himself. After Vader's death, Palpatine would not risk living with another apprentice after losing three, and so the Dark Side Elite became the right hand of Lord Sidious during Operation Shadow Hand. Palpatine also trained several Emperor's Hands, special assassins that did his bidding. Famous ones included Darth Vader's daughter-in-law, Mara Jade Skywalker, and Shira Brie, who would later become Lumiya, Vader's apprentice. Mara was later redeemed by her future husband, Luke Skywalker, and became a Jedi Master.
After the Battle of Endor, Dark Jedi sporadically appeared throughout the years, until the creation of the Second Imperium. Many Dark Jedi, led by a former Jedi named Brakiss, assaulted Luke Skywalker's Jedi Academy on Yavin 4. Brakiss was eventually killed when a Royal Guardsman triggered an explosive device after nearly being killed by him, thus destroying the Second Imperium.
In the Empire Reborn campaign, Lord Hethrir, Desann, and Admiral Galak Fyyar created an unusual hybrid of Dark Jedi and stormtroopers by giving them cortosis-weave armor and crystals from Artus Prime. These soldiers were called the shadowtroopers. Jedi-in-training Kyle Katarn and his partner Jan Ors put a stop to the project and defeated two of the three heads.
In 14 ABY, Master Katarn and his new apprentices, Rosh Penin and Jaden Korr, came to the New Republic's rescue as Desann's former apprentice, Tavion Axmis, discovered the Scepter of Ragnos and created a new Sith cult called the Disciples of Ragnos. The cult consisted of cultists and Desann's old Reborn warriors, and was also allied to the Imperial Remnant. The Disciples and Tavion were defeated by Korr and Katarn, and Rosh was redeemed after joining.
During the Yuuzhan Vong war and the Dark Nest crisis, scarcely any Dark Jedi remained. The only ones seen during both these crises were Nightsister Lomi Plo and Shadow Academy student Welk. Both were killed by Luke Skywalker during the Dark Nest crisis. Also, a former Jedi named Alema Rar became a Dark Jedi during the Dark Nest Crisis, but was supposedly killed during one of its final battles. Alema was later proven alive but grievously wounded, only to die later by the hands of Jagged Fel.
Despite all these tragic deaths and defeats, the Dark Lady Lumiya eventually resurfaced, but was eventually killed by Luke Skywalker, who believed that she had murdered Mara.
While Jedi were capable of falling to the dark side, it was also possible for a Dark Jedi to be brought back to the light. Notable examples of redeemed Dark Jedi through history, include: Anakin Skywalker, Juhani, Yuthura Ban, Atris, Quinlan Vos, Yun, Kyle Katarn, Mara Jade, Zekk, Rosh Penin, Kyp Durron, Flint and Kam Solusar.
A great deal of controversy has arisen over the issue of who is and is not a Dark Jedi. Perhaps due to uneven usage within canonical sources, many fans have argued that Dark Jedi describes only those dark side users who had previously been a Jedi or were at some point in the process of being trained as a member of the Order. Leland Chee cleared up the confusion in two posts on the StarWars.com forums:
Dark Jedi (also "Fallen Jedi") was a term typically used to describe Force-sensitives, mostly former members of the Jedi Order, who embraced the dark side of the Force, though usually, Dark Jedi was a common name for the ones who did not possess the teachings and characteristics of a Sith. The first Dark Jedi was believed to have been Xendor, but with the discovery of a Holocron from a Dark Jedi that lived in 27,000 BBY who was Adamin Cay-Band, also known as Darth Asmodeus, and the few surviving books on the teachings of Erik Wolfen, it is very possible that he wasn't.