Master of the Peak - 25G - Complete "Soldier's Peak" (Warden's Keep DLC)
This really speaks for itself, once you have killed Avernus and his Shambling Skeleton's at the end of the DLC, Levi will start talking to you and just before he says a word, the achievement should unlock, just to remind you as well make sure you get the Diabolist achievement before finishing the DLC!
Otherwise you will have to make a new character and start it again.. If you're just going for the achievements this isn't nice but if you're a DA fan and you don't mind playing all of DA again you wouldn't mind doing it again like me. (Try to get both achievements first time like I did) (: