Xendor

Xendor was a Kashi Mer Jedi Knight around the time of the Great Schism, and was also believed to be the first Dark Jedi.

Biography
The Force was strong with Xendor, unfortunately he used that power for evil. He used the Bogan or otherwise called the dark side of the Force to reinforce his jar'kai techniques in combat, causing him to be exiled by the Jedi Order. After being banished, he gathered a large following of other Dark Jedi, known as the Legions of Lettow. Xendor was believed killed by the Jedi Order, and leadership of his Legions was briefly picked up by his lover, Arden Lyn.

Xendor's successors later conquered the Sith species, becoming the Sith Lords. As the first Dark Jedi, Xendor is the predecessor of the Dark Lords of the Sith, and is sometimes counted as the first. Since the Sith Empire was formed in 6,900 BBY (18,000 standard years after his death), he was never a Sith Lord. However, he played a fundamental role in the formation of the Sith Order.