After you gain the battle tower, the surrounding area becomes covered in a poisonous fog, it's at this point you can gain this achievement.
Go into dragon form and fly towards the top right corner of the map, almost off the map ad fly slightly to the south. This is where you'll find a raft with a diary on it, read it and the ahievement will unlock.
This raft can be quite hard to find and may be confused with a wrecked boat further south. It is a quite small raft stuck against a rock.