Set foot in every area in the game
This has to be done in one playthrough. You'll need to hit every location on the main map, and the sub maps for the Deep Roads and Denerim. You'll need to do every main quest and quite a few side quests to visit all of them. Make sure you go into every door, every shop, every section every town, every Chantry...everywhere. These are the locations you'll need in your Codex.
See this thread for a list of the locations you'll need for this achievement.