I did the betrayal quest line back in wrath and never bothered doing cleansing drak'tharon. Now I decided to go back and get the achievement. So I picked up the quest cleansing drak'tharon and got to the scene where the LK apears and stuff, after that I picked out my reward but I didnt get the achievement...
and ofcourse no follow up quest, anyone knows what to do next?
There are some random quests that start from mob drops but usually don't account for those achievements hmmm dungeon quests? maybe a quest chain you missed?