This Datacron unlocks the codex entry - Galactic History 86: The Empire's Revenge
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 Republic had enjoyed three hundred years of peace, knowing nothing of the Sith Empire's continued existence on Dromund Kaas. When strange battle cruisers were reported in the Tingel Arm, the Republic sent a diplomatic vessel that was annihilated after sending a frantic transmission. As the Republic and Jedi began to investigate these mysterious newcomers, a massive Imperial battle fleet descended on Korriban, triumphantly reclaiming the Sith holy world for the Empire.
Imperial fleets converged on targets in the Outer Rim, devastating key military outposts. Planetary governments corrupted by secret Imperial agents suddenly revealed their allegiance to the Sith. No one knew how powerful this reborn Sith Empire was, how large it had grown during the past millennium or where its capital was located.
The Republic was thrown into chaos. Disagreements in the Senate and the defection of key systems delayed its military response, while the Imperial forces coolly carried out the Emperor's plans. Despite several important victories--most notably at Bothawui--the Republic was forced onto a defensive footing. The first stage of the Emperor's plan was a success.
This Datacron is located in the Imperial Base Camp of northern Ilum. Much like the Yellow Matrix Shard piece is, this Datacron is protected by a member of the Imperial faction (the Yellow Matrix Shard it was protected by a Republic mob, but you get my point...) Despite being in the middle of an Imperial base this Datacron is still rather easy to get if you're Republic, even without Stealth. (Screenshot 1) has a look at the Datacron from a distance, the frozen lake nearby the base.
If you look close enough you can see the Datacron inside the hut directly in front of me. From where I am now it's pretty much a straight run in to get the Datacron if you want to avoid all of the Imperial forces. To see the exact location of the Datacron on the map and in game take a look at (Screenshots 2 & 3)