Loathsome Rat Men (WAR History and Lore)  

Located under the bridge just outside of the Wolfenburg Inn, at approximately 52k/52k. Move to the south side of the bridge on the western bank before the water's edge and you should see a grate under the bridge with a Dead Skaven caught inside.

Interacting with the body will grant you the unlock.

History & Lore in Ostland