Risen

Beginning in Chapter 2, you will meet Balturo inside the Don's cave who tasks you with helping Patty find her father's treasure. Find Patty down by the beach near the Harbour Town secret entrance, and then follow her up to near the bandit camp where you will dig up the first treasure. Patty will stay behind while you locate the remaining chests. (list?) Once you have all 5 opened, and the series of clues, head back to where you left Patty to find her missing. She has been captured by Romanov. Talk to Eronga and he will lead you to the western ruins, and from there trail all the way to a small ruin at the end of the western coast. You will meet with Romanov and presented with a bit of a choice.

 

You can trade Steelbeard's treasure for Patty, or you can say forget about Patty and keep everything but a map for yourself, but this doesn't help towards the achievement. The third and best option is to simply take on Romanov and his lackeys, keeping the treasure and the girl. Romanov will have the key to Patty's cell on him, so head back to the larger ruins Eronga left you in, and look for a staircase leading down, towards the outer wall of the ruins, towards the ocean. The door is to your left as you head down the stairs. Find Patty inside, completing the quest "Where is Patty?," and this achievement will pop.