Dark Jedi were Force users, often former and fallen Jedi, who followed the dark side of the Force but were not Sith. Notable examples included Xendor and his Legions of Lettow, the Dark Jedi of the Bogan, and the Bogan Empire. The Guardians of Lettow were a later Dark Jedi group.
Thousands of years later, another schism in the Jedi Order led to the creation of the offshoot order called the Ospion Guardians. The Ospion did not practice the dark side at first, but many later chose to when they discovered the teachings of Xendor on Conscio. A civil war amongst their ranks saw those who followed the dark side defeat those who did not, creating the Dark Jedi of the Bogan.