As Al Quelt Moreza can be very crowded, it is recommended that you venture a little deeper into the dungeon to have a better chance of camping a room for monsters. Al Quelt Moreza, Narthex is ideal, as there is no Siegmund spawn to interrupt you. The 20 Pieces of Sulfur are a quest item, and as such will not appear as a drop. To check how many you have obtained check your Quest Log.