This Datacron unlocks the codex entry - Galactic History 87: A Wayward Apprentice
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.
This Datacron is located in far northwestern Ilum but to get to it you will need to go down an icey slope in northeastern Ilum (Screenshot 1 has it marked). Where the big white X is on the map is where you will find the slope. You can actually kind of see the slope on the map if you look close enough. Once you're down into the icy chasm you'll want to run all the northwest to where I am on the map in (Screenshot 2). Here you will find an entrance to a cave with a lot more of those Lisk mobs as well as a big champion mob called The Crystal Eater.
All the way in the back northern end of the cave you'll find the Datacron. If you're looking at the Crystal Eater it's to the left, or, in otherwords, straight ahead from where you enter the cave.