Xendor was the first Jedi (appx 24500yrs before the battle of Yaven) to fall to the dark side. (First Great Schism) Many years later (18000yrs later that is) many Jedi fell to the dark side and and had a war with the "Good" Jedi (Second Great Schism) which lead to the Hundred Year War. When the war ended Ajunta Pall was one of the many dark jedi involved. When the dark jedi were defeated during the battle of Corbos ( 6900yrs BBY)the Dark Jedi were captured and exiled way from the republic. They landed on a either Ziost or Korriban (never truely said). The Sith (the natives of the planets) bowed to the Dark Jedi and thier powers. With Ajunta Pall being the strongest of the Dark Jedi he became the First Dark Lord of the Sith. So there you go. Ajunta was the First Dark Lord. The were others before him like Xendor but since the Sith were not found yet he was the leader of another cult of dark users called the Legions of Lettow