As you enter this place, head south as far as you can and grab the Treasure Chest you see to get an Eye Drops. Next, head north and grab the 2 Treasure Chests here to get 2 Hi-Potions, then, head southwest and grab the Treasure Chest here to get an Ether. Afterwards, follow the path west and head southwest before you head north to find a Treasure Chest with a Silk Web. Then, go ahead and go north and follow the path to find some stairs, so, head down them and on this floor, follow the path north and grab the Treasure Chest near the stairs to get a Slumber Stalk. After this, head down the stairs nearby and on this floor, head south and grab the Treasure Chest here to get a Silk Web, then head down the stairs to the south.
On this floor, head north and head up the stairs here to reach an area with a bunch of primitive people here. None say anything useful, but the one coloured red with a green mask sells weapons, Armour and Items. Don't bother, though, as nothing he sells is that good. Anyway, exit this area and head back up to the floor where you got the Slumber Stalk from and once there, head east and grab the Treasure Chest here to get a Poison Scroll. If you don't have it already, definitely use it, as it's a great spell to use and afterwards, head south as far as you can and open the Treasure Chest here to get a Blind Scroll. Feh, worthless. Next, go west and grab the Treasure Chest here to get another Poison Scroll, then continue to the west to find another Treasure Chest which holds another Blind Scroll.
Next, head north and go down the stairs and on this floor, head south to find another Treasure Chest which holds a Garlic, then head north and follow the path around and go down the stairs you see. On this floor, head south and head up the stairs you see and on the previous floor, head north and open the Treasure Chest you see to get a Phoenix Down, which is guarded by Ghasts, Ghouls, Shadows and Wraiths. Next, head back down the stairs and on this floor again, head east and go up the stairs here and on this floor again, follow the path and head down the stairs you see. On the next floor, head west and open the 3 Treasure Chests which holds 2 Slumber Stalks and a Gaia Drum, which is guarded by Poison Toads and a Wild Horn.
After this, head back up the stairs here and then down the next stairs on the next floor and on this floor again, head northwest and go down the stairs here and on this floor, grab the Treasure Chest here to get a Thanatos. Then, head back up the stairs and once again in this area, head east and grab the Treasure Chest you see to get 1,000 Gil. After this, continue east and go down the stairs here and on this floor, grab the Treasure Chest nearby to get a Sleep Scroll, then head southwest and enter the door. Inside, you'll see the Boss nearby, so, if you want a strategy for this, head to Section VII. Boss Guide now, otherwise, fight this yourself and then move on.
After the Battle, open the Treasure Chest here to get the Black Mask! Then, just head north and step on the teleporter to leave this place. Once you're outside, head back to Mysidia and sell anything you don't need, stock up on what you do need and then heal up at the Inn. Afterwards, leave Mysidia and check your Map. You see that circle of brown stuff literally right next to Mysidia? Well, it's a ridge of mountains and in the middle is a yellow square. That's where you need to go now. I suggest you use your Ship to go over to the other side, since you have to enter the place from the northeast, so, go ahead and do that. Once there, enter the cave.