This Datacron unlocks the codex entry - Galactic History 69: The Mandalorian Wars
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 Mandalorians had returned to take revenge on the Republic and sate their lust for war and conquest. With the Jedi Council refusing to lend military aid--believing the true threat was still unclear, and the risk of conflict-hardened Jedi falling to the darkness was high--the Republic was unable to prevent the Mandalorians from devastating the planet Cathar. Millions died. This was the last straw for several young Jedi, who disobeyed the Council to join the war effort.
The most notable was Revan, a charismatic and skilled leader who rose to the rank of general. Revan's friend Alek, later known as Malak, distinguished himself on the front lines. Revan's tactics and Malak's unyielding strength turned the tide, helping the Republic forces to victory at Althir and Jaga's Cluster.
But the Council did not approve. Seeing their rash involvement in the war and their gradual adoption of Mandalorian tactics, the Council watched Revan and Malak's heroics uneasily, remembering the fall of Exar Kun.
This is one of the easiest Datacrons to reach on Belsavis, the only problem is it is guarded by some difficult mobs to kill so if you aren't at least level 50 you will want to bring a friend. To get to this Datacron you will need to go to the far northern section of the High Secruity Section (Screenshots 1 & 2). There is a small path down that leads to a cave which is where you will find the Datacron. Now as I mentioned before getting to this Datacron is difficult, it is guarded by a level 41 champion and other elites inside the cave.
At level 50 I was able to solo all of the mobs in the cave including the level 41 Champion without dropping below 90% health, so unless you're really really bad this Datacron will be cake to get.