Just like any other map completing achievement, I've uncovered all the dark areas on the map. But did not receive achievement Mapping the Realm: The Diadem.
Even went inside the emergency mission area where you get teleported, but was unable to defeat the mission itself if this is reason of the cause or not.
World name: Leviathan
Character name: Othinus Veratyr
In-Game Time: Since 3.55b
Area and Coordinates: Diadem