The chest are guarded by mobs. Spiders and rumbler types. If a gnoll spawned behind the explodable wall you will have a key that can open only one chest then it despawns. Heard coin jingle sound on chest i opened. Note you are below the area where the two phantasmials are sometimes they will come down and you can kill them here and the barrel will be left up top. Also you can pull almost all the sulfer emissions down to you.
