Khorne (WAR History and Lore)  

on the Order side of the map, southwest of the Gore Wood Public Quest there is a large tent (6.5k/58k), surrounded by Chaos forces. Outside the entrance to the tent, right next to what appears to be a torture device in the shape of a Chaos symbol, there is a Powder Barrel you can interact with. Doing so will grant you the unlock.

History & Lore in Ostland