The Traitorous Ones were a Sith organization founded in 120 ABY by the deceased Nat-ahn Mandrex and his last surviving followers. He had felt what his son had created and decided to make his own group to counter it.
The main goal of the Traitorous Ones was to find a way to restore Mandrex back to his former state and to disrupt his son's plans in the process.
In 1,119 ABY, they were finally able to recreate Mandrex, at the cost of their lives. (A fact that he had failed to reveal to them.)