Dark Elf Treachery (WAR History and Lore)  

At around 8k/20k, just off the small path that branches off the north-south running main road, on the opposite side of the map from the Spires of Narthain Public Quest, there are three short broken down walls that roughly form a triangle. Near the base of this triangle is a busted bolt thrower, and directly beside it is A Discarded Message.

Interacting with this parchment will grant you the unlock.

History & Lore in The Blighted Isle