This prophecy did not have any official scripture outlining it, nor do I know where it originated. It could have been a scheme by Markos Ragnos himself, instead of a prophecy, for all I know. Either way, the events transpired. The prophecy was first made sometime during the Great Hyperspace war.
It foretells the return of the Dark Lords of the Sith. Naga Sadow seemed to be keen to the prophecy, following his defeat on Korriban. If my information is correct, he created a Sithspawn to guard against them. I originally learned of this prophesy from the Massassi warriors who pledged themselves to bloodline. This prompted me to search both the Sith and the remaining Purple Lotus records for more information. The reason I did not learn of it prior is due to it having already come to pass, but even history and past prophecies can aid us in the future.
Those that fulfilled the prophecy were Exar Kun and Ulic Qel-Droma. They once battled each other and each had a Sith amulet on their person. While battling, they had a reaction and the two saw visions of the ancient Sith empire and the Dark Lords of the Sith. Markos Ragnos appeared to them both, marking them and naming them the master and apprentice, the new Dark Lords.