The Temple of Khaine (WAR History and Lore)  

This unlock is located on the Destruction side of the map, north of Burnwood, the Dark Elf Chapter 10 camp. Around 9.2k/4.5k you will find a Defiled Tree Trunk lying on the ground near some Avelorn Archers. Interact with the tree trunk to obtain this unlock.

History & Lore in Avelorn