Achievement name: The Damned One
Until you complete the quest 'To Lay a Ghost to Rest' you are confined to the North-East part of Broken Valley - Rhode's men will stop you going any further. Once you complete that quest the route is unlocked allowing you to venture onwards.
When you do venture slightly further (in fact just around the corner...) there will be a couple of cut scenes which culminate in a visit from Damian. Once the cut-scene ends, the achievement will pop.