Codex/Galactic History 78: The Sith Triumvirate

The Galactic History 78: The Sith Triumvirate codex entry is located within the Achievements:Datacrons section of a player's codex once it has been unlocked.

Codex entry
"This datacron holds unheard of power and knowledge collected by an ancient race. You access its power and discover writings which are clearly only one small piece of a massive galactic history:

In the chaos after the Jedi Civil War, new Sith began to rise up, inspired by the likes of Exar Kun and Darth Revan and unaware of the Sith Empire still hidden on Dromund Kaas. Three lords stood above all others: Darth Nihilus, whose hunger devoured all life around him; Darth Sion, whose shattered body was held together by hatred; and Darth Traya, once a Jedi Master but now the Lord of Betrayal. These Sith Lords ruled from ancient Sith ruins on Malachor Five.

Darth Traya was the most senior of the three, but when Sion and Nihilus united against her, she fled Malachor Five and took the name Kreia. Darth Sion created an elite cadre of Sith assassins, eroding the already-weakened Jedi from the shadows, while Darth Nihilus began roaming the galaxy, seeking the life energy of Force users to satisfy his insatiable hunger. The Republic’s short reprieve was over."