It can be found triangulated Northeast of Camp Sentinel and South of the Depleted Ore MIne. Once inside you'll find a dead priest and by him a Book of Notes and a key. Upon reading it you find that the bandits are after something, but he does not know what and obviously never finds out. Down some stairs you will find a group of bandits one name Sonja who drops another key. After the fight you can go right or left, if you go right you find a barrier in the way and three bandits on the other side.