Hi, The obsidian chest is not part of the Mossback quest. You don't need to do quest for that. The obsidian chest are part of a DS quest to get the dragon staff, which enables you to fly a drake to fight Malistaire. You do however, need to do all the Mossback quest if you are a life wizard. Those quest helps you get your lvl 48 life spells.
If Mossback isn't there (unless you are in the Tree of Life) switch realms. That normally helps.