You can see this datacron up on a mushroom while exploring Belsavis, but you will need to find another way up to it.
Quick Travel / Taxi
Find the Secret Entrance
Follow the Path
Follow the grassy path to the datacron.
Belsavis Purple Mastery Datacron Guide
The Belsavis Purple Mastery Datacron is located in the High Security Section map on Belsavis grants Mastery +4 (it used to grant Willpower +4 before the stats changed).
The Belsavis Purple datacron is in the grassy plateau accessed by a hidden path and is available to both Imperial and Republic players. Finding this datacron will give you a permanent stat boost of Mastery +4 to all your characters on your server, and will unlock the Galactic History 72: The Fall of the Covenant codex.
Belsavis Mastery +4 Datacron (Both Factions)
Quicktravel: 
Republic Watchtower / 
Imperial Command Post
Taxi: Republic Watchtower / Imperial Command Post
Location: High Security Section - grassy plateau accessed by a hidden path
Difficulty: Easy - can walk right up to it
Codex: Galactic History 72: The Fall of the Covenant
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