Ajunta Pall
"This dark Jedi dominated the Sith people, though his methods of doing so were not recorded in 5adow's Holocron. He turned them into unwilling thralls, stealing their knowledge of sorcery and using it against them. This Jedi proclaimed himself the Dark Lord of the Sith, making him the first in a lineage that would plague the rest of the galaxy for untold millennia."
Source: Tales of the Jedi Companion