The Mysidian Cave has things all backwards. There's great treasure right on the first floor, which gets less impressive as you descend. In fact, you don't even need the Black Mask to retrieve the Black Garb and Gold Hairpin, though you will need it to make it to B2 and beyond. The four weapons found here can all be purchased in Mysidia, and money is likely no object at this point. The real prizes here are the Drain and Osmose Tomes, though. Osmose in particular effectively gives your black mage unlimited MP, and changes your whole strategy for the game.