Dark Jedi

Dark Jedi were Force-sensitives, frequently Jedi, who chose to deny the light side of the Force and follow the dark side. 
Although Dark Jedi originally referred to "A Jedi who fell to the dark side", it also refers to uninitiate Force-sensitives who received no Jedi training and began their careers under another Dark Jedi. In some cases Dark Jedi also included "artificial" Force-sensitives who served the dark side, such as the Reborns and the Shadowtroopers. 
Among the Dark Jedi, Xendor and Lettow were believed to be two of the very first to ever exist in galactic history.
==Relationship between Jedi and Sith==
Dark Jedi were not the same as the Sith, though the first group of them were the predecessors of the Dark Lords in the ancient Sith Empire. Despite the fact that both groups called upon the dark side of the Force, the Sith have their own codes and history, study their own skills and techniques, and possess secrets like Sith alchemy. Furthermore, the Order of the Sith Lords were limited to two members, a master and an apprentice, whereas Dark Jedi bear no such limitation. Many dark side practitioners, such as Count Dooku, considered that the power of a Dark Jedi is nothing compared to the power of the Sith. Some individuals, such as Darth Vader, practiced both Dark Jedi and Sith techniques.
== History of a Dark Jedi ==
It is said that after the Dark Jedi were created, the Sith were soon born. Many Jedi seduced by Exar Kun became Dark Jedi. After the death of Kun, many Dark Jedi still exsisted and served under new Sith Lords like Darth Malak and Darth Nihilus. During the era of Darth Bane, many dark siders were Sith Lords and very few Dark Jedi were ever seen until the Clone Wars.
After the death of Darth Maul, Darth Sidious apprenticed Count Dooku who also aprenticed several Dark Jedi including Asajj Ventress and Sora Bulq. He also supposedly had under his wing the Jedi-turned-Dark Jedi, Quinlan Vos. Ventress and Bulq never worked together quite often since she mostly worked with a Gen'Dai bounty hunter named Durge. Near the end of the Clone Wars, every thing went wrong for Dooku. Ventress, after faking her death, fled to an unknown place. Sora Bulq was killed by Quinlan, after turning back to the light, during the Seige of Salecumi along with another Dark Jedi, Skorr. A few months after the seige, the Count was killed by Anakin Skywalker on General Grievous's flagship, The Invisible Hand. Soon after, Darth Vader became Palpatine's newest apprentice and the Sith ruled the Galaxy once more. This would lead to many more Dark Jedi. 
 
During the span of the Galactic Empire, Palpatine established his own Dark Side Elite forces which were composed of Dark Jedi. As Palpatine did not recruit a new Sith Apprentice to replace Darth Vader, the Dark Side Elite became the right hand of Lord Sidious during Operation Shadow Hand. He also trained several Emperor's Hands, special assassins that did Palpatine's bidding. Famous ones include Mara Jade and Shira Brie, who would later become Lumiya, Darth Vader's apprentice.
After the Battle of Endor, Dark Jedi sporatically 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. He was eventually killed when a Royal Guardsmen trigged a explosive device after nearly being killed by Brakiss destoying the Second Imperium and killing Brakiss.
During the Yuuzhan Vong war and the Dark Nest crisis, scarecly any Dark Jedi remained. The only seen ones during both these crisis' were Nightsisters Lomi Plo and Welk. Both were killed by Luke Skywalker during the Dark Nest crisis. Also, Alema Rar, a former Jedi, became a Dark Jedi during the Dark Nest crisis but was killed during one of the final battles of it. 
However, Lumiya may have surfaced again. This however may or may not be true.
==Path of Dark Jedi==
Other than the Sith, Dark Jedi in galactic history were loners. Seldom did a group of Dark Jedi form a well-organized group in order to expand their following or reinforce their strength. The Sith often seduced Dark Jedi to work for them, without granting them any knowledge of Sith techiniques. Yet most Dark Jedi were willing to serve the Sith, seeing 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.
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 Bastila Shan, Quinlan Vos, Luke Skywalker, Kyle Katarn and Kam Solusar.