To be united by hatred is a… fragile alliance at best.
~ Darth Traya on the Sith Triumvirate
The Sith Triumvirate is a loose alliance comprised entirely of Sith lords and serves as the main antagonist faction of Star Wars: Knights of the Old Republic II - The Sith Lords. Formed from the remnants of Darth Revan's Sith Empire, the Sith Triumvirate was founded and originally lead by Darth Traya who ruled alongside fellow Sith lords Darth Nihilus and Darth Sion prior to their betrayal of her.
Following the formation of the order, this dark Sith organization waged a bloody war across the galaxy where they destroyed the Jedi Order and nearly exterminated the Jedi from the galaxy had a generation of them, the Lost Jedi, not survived. The organization serves as the primary antagonists in the game and is responsible for the entire plot.
The Sith Triumvirate was an alliance comrpised and lead by the three Sith lords, Traya, Nihilus, and Sion. All of them were Dark Lords of the Sith and eached bared a title to represent their mastery and skills of the force.
Darth Traya, Lord of Betrayal
Darth Traya was previously a Jedi Master named Kreia and was a very contemplative and philisophical woman as a member of the Jedi Order to the degree that she was considered to be a Gray Jedi.