This Datacron unlocks the codex entry - Galactic History 47: Naddist Rebels
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:
As the Jedi know only too well, evil can arise from the most unlikely of sources. In the years after the Great Droid Revolution, two aristocrats, Satal Keto and his cousin Aleema, came to power in the Empress Teta system. Bored and spoiled, they became interested in the secrets of the Sith. They and their friends formed a Sith cult named Krath.
But Satal and Aleema began to crave real power. During a visit to the Galactic Museum on Coruscant, they spotted a book of ancient Sith lore and rituals and decided to steal it.
Once they had the book, they realized it was written in a long-forgotten Sith language. But Satal learned of the planet Onderon, once ruled by the Sith Lord Freedon Nadd, and the cousins left to unlock the book's secrets there.
This Datacron is located in Castle Panteer which is located in far western Glarus Valley. If you're having trouble finding it, it is where the final quest of Alderaan takes place. In Castle Panteer there is a small little gap between the door and a wall which you can sneak through to reach the Willpower Datacron (Screenshots 1 & 2). Once you make it through the gap head up the stairs and follow the northern path around the room you come out into. The reason you want to do this is because there is a fallen pillar which blocks off the southern route.