Myrmidia (WAR History and Lore)  

This unlock is well inside the RVR area, so if you are over-leveled for Tier 3 RVR, be prepared to be chickened!

Enter the gate to Hallenfurt Manor and immediately take a left, following the outer wall. At the base of the wall, at one of the wall supports, there will be a pile of books, one being named On Warfare. There should be three trees to your right and a sheer rock wall in front of you around 46k/61k.

Interacting with the books will grant you the unlock.

History & Lore in High Pass