The title of dark lord of the sith was brought into effect under Exar Kun. He was the Dark Lord for a really long time (depending on who you ask at least.) his spirit was excised from the temples on yavin where Luke set his Academy up. Those were his personal playground. Anyway either you believe that he kept the title for 5000 years and thus negate all intervening Dark Lords. (meaning that Reven couldn't be Darth Revan and so on) or he lost his title along with his body. Now Vader is called the Dark Lord because he is, for all intents and purposes, The strongest sith. The title of Dark Lord goes to the Strongest Sith until he dies, Usually from his apprentice. The other major player during Exar Kun's reign is Ulic Qel-Droma, this guy was named First apprentice. meaning that you can technically have a dark lord academy. but you only work one on one with the first apprentice. Course this totally invalidates the Academy on Korribon from Kotor but who cares.
Hee Hee