Equip Griz and head down to Demon's Delve. Griz will guide you to an empty sack ( 91, -67, 119 ) Copy/waypoint 91, -67, 119 it is in the back of the cave, on a little table.
Griz will guide you to the NW tunnel, where there are two carts in the cave ( 243, -53, 51 ) Copy/waypoint 243, -53, 51. One of the carts contains a black crystal (move around the carts - when in the right area the dialog will begin, then click through Griz's chat dialog to receive the update).