I think I solved this. Either I overlooked the wording, or the wording on the achievement has been updated, but it says that you must have map completion on Dragon's End; as in the actual map. I'm not sure if the wording was different before or not, but I took it before as meaning any End of Dragons map. I haven't completed Dragon's End, yet. Can anyone confirm that they have used a Dragon's End Willbender's cache, specifically?