Codex/Galactic History 87: A Wayward Apprentice

The Galactic History 87: A Wayward Apprentice 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:

The Sith Empire was founded on absolute obedience. As the war between the Republic and the Empire raged, however, the Emperor sent out an order: His apprentice, Exal Kressh, had abandoned her duties and disappeared, and for this treachery she was to be destroyed.

The Dark Council dispatched the young Sith Teneb Kel to hunt Exal Kressh. He followed her to Lenico, where a vision showed him the Emperor’s true plans; plans Exal had rebelled against. As a Republic fleet used information supplied by Exal to attack Korriban, Teneb Kel and Exal Kressh clashed in Korriban’s tombs. With the help of his slave Qawohl, Teneb cut Exal down and the Republic attack was repelled.

Teneb Kel reported to the Dark Council and struck a bargain: make him a Darth, and he would share what he knew of the Emperor’s intentions. Teneb chose the name of the guide from his vision on Lenico, Darth Thanaton."