Triumvirate' is a term used to describe the Three Masters who united on Antei to form the basis of the Star Chamber, some three thousand years before the Battle of Yavin. The Sith King of Antei, the Warlord Okemi was the first of the three. He was joined by the Sorceress and Marked of the Wanderer, named Tiamat of the ancient Krath Order, and later by Ferran, a Mecrosa Assassin from Corellia.
Trivia
The term Triumvirate, and its three leaders are a non-canon part of Brotherhood lore created by Trevarus Caerick as a mythological basis for the Star Chamber. They were introduced during the Sixth Great Jedi War.