The Dark Jedi have fallen out of Star Wars canon, but they used to be an integral part of the lore. Before the word "Sith" became a common name for Dark Side Force-wielders, many antagonists such as Palpatine and Darth Vader were called Dark Jedi. Let's take a look at the history of the term and some notable Dark Jedi throughout Star Wars Legends.
"Dark Jedi" was coined by Timothy Zahn in his 1991 Legends novel, Heir to the Empire, which described any Force-user who fell to the Dark Side or wasn't trained by the Jedi Order. At the time of the novel's release, the term "Sith" hadn't been clearly defined yet, and Zahn tried to limit his use of the phrase "Dark Jedi" throughout his work. While the script for A New Hope and some Legends novelizations referred to Darth Vader as The Dark Lord of the Sith, other characters such as Palpatine were frequently called Dark Jedi. Sith fell into popular lexicon following the release of The Phantom Menace, which gradually phased out the term Dark Jedi.
The history of Dark Jedi in Legends begins with Xendor, a former Jedi Knight and member of The Kashi Mer Dynasty. During his time as a Jedi Knight on Ossus, Xendor began to speak out against the Order, who were closed off toward the study of Bogan, an early term for the Dark Side of the Force. After the Jedi High Council denied Xendor's request to open an academy to teach Bogan, he quietly broke off from the Order and established an academy in Lettow. Xendor and his lover Arden Lyn would go on to form a group of Bogan wielders called the Legion of Lettow, who entered into an open war with the Jedi Order called The First Great Schism. The legion was ultimately defeated by the Jedi at Columus some time around 24,500 BBY.
Ajunta Pall became the next prominent Dark Jedi in Legends and is responsible for their integration into the Sith Order. After attaining the rank of Jedi Master, Pall began to practice alchemy and learned the secret of creating and shaping life. The Jedi Order became fearful of this power, and ousted Pall and his followers from their ranks. Pall would eventually declare war on The Jedi Order in what became known as the Hundred-Year Darkness, but his side was ultimately defeated during the Battle of Corbos.
Following their defeat, Pall and his exiles discovered the Sith world of Korriban on the Outer Rim. During this time, the Sith were an alien species native to the planet, who were adept in the Dark Side of the Force. Pall's exiles were surprised by the species expertise with the Dark Side, and soon conquered the planet. These Dark Jedi would eventually enslave the native Sith population, while Pall became the first Dark Lord of the Sith and established the First Sith Empire some time around 6900 BBY.
Following Pall's conquest, the cultures between Dark Jedi and Sith began to merge into one identity, but they avoided all-out war with the Jedi until the rise of Darth Naga Sadow. A half-breed Sith from Ziost, Sadow attempted the First Sith Empire's conquest of the galaxy during The Great Hyperspace War, but was ultimately defeated by the Republic and Koros in 5000 BBY. Most of the Dark Jedi who arose after Sadow took up the banner of the Sith Empire, who would wage numerous wars against the Jedi and the Republic for thousands of years to follow.
While most Dark Jedi were loyal to the Sith Empire, not all of them were Sith adherents. Allya, a rogue Jedi Knight, was expelled from the Order and fell to the Dark Side, but did not join the Sith Order. The Dark Jedi was exiled to a penal colony called Dathomir, where she encountered other exiles and taught them to use the Force. These women became known as the Witches of Dathomir, who split off into different clans. Each of these clans held different rules regarding the Force, but were not allowed to use it for evil, as their founder Allya eventually learned about the error of her ways.
One of these sects known as the Nightsisters took in Asajj Ventress in Legends after Palpatine and Dooku attempted to kill her. Ventress was also a Dark Jedi, and while she was Dooku's acolyte, she was never allowed into the Sith Order. Following the massacre of the clan, Ventress went into exile and decided to change her path by becoming a bounty hunter.
Although the term Dark Jedi isn't used in canon anymore, its lengthy history holds a lot of importance in Legends.