The Overlords (the Overlord of Kressh, the Taral'ari; and the Overlord of Ragnos, the Maleficarum) are the patriarchs of the two Sith cults aligned with the Disciples of Sadow. The two lines stretch back thousands of years to the the last days of the original Sith Empire which saw the deaths of Dark Lords Marka Ragnos and Ludo Kressh.
After Astronicus Dlarit learnt of his heritage as the last surviving descendant of Naga Sadow, his search for followers led him to the Disciples of Ragnos and Kressh. The two cults, at the time led by Ximeno and Eagleye respectively, swore fealty to the young Sith heir so that, together, the three lines could work towards the reestablishment of the true Sith Empire.
Maleficarums
When the Krath oracle Trevarus Caerick appeared, divining the tenets recorded in the Chronicle of Dark Souls, which would later become the doctrine of the Final Way, the Disciples of Ragnos hailed his connection to the spirits of old and anointed him their new overlord, their Maleficarum, believing his divine providence would usher them into the next Golden Age. Years passed as the Dragon Shan Long festered beneath the surface, battling against his alter ego, until in 29 ABY the oracle revealed his true allegiance, betraying the Disciples. When the Shan Long devoured the sorcerer's soul, the Dragon emerged as the true source of his visions all along, and Trevarus Caerick, the Deceiver, faded into the annals of history, being remembered only as a traitor, allowing Shan Long to at last be hailed as the voice of the Maleficarum.
Taral'aris
The Disciples of Kressh, or the Jentarali, the hidden guardians, as they are also known, have long been led by their strongest champions, the greatest of which is the Taral'ari, the overlord of the guardians. When Prince Sadow found the stranded cult near Rhelg, they were then led by the Taral'ari Eagleye. During the years that followed, Xanos Zorrixor emerged to lead them through the Exodus and the battles that followed, being declared the new Taral'ari. However, during the Yuuzhan Vong War, the Taral'ari was twisted into Darth Vexatus, the Betrayer who joined the false Maleficarum Trevarus Caerick. After the Taral'ari's treachery, none amongst the Jentarali rose to become the new overlord, the hidden guardians fearing the name was now tainted. For years the seat sat empty, until finally, in 33 ABY, Kharon Daragon rose to take up the mantle of Taral'ari.