Get here by either summoning Flammie and landing in front of the cave, or walking that short, short distance on the Mountain Road. In the first room, defeat the Fierce Head. In the next room you can go left or right; take the right path after you give the Dark Knight a beating. Go north and ready the Axe. Whomp the stalagmites to your right; left is ignored for now, but you have to return here because there is a crack in the wall. Go south for now.
Go to the left and through the passage. Tap the switch for a pole to appear. Return and take the path leading south. In the next area, walk left and step on the switch. This opens up the gate you see in the lower-left corner of the screen-the chamber that is now accessible contains a Glove's Orb. Ignore it for the moment. Take the stairs leading to the floor below and go through the passage.
Watch out for the Dark Knight and walk through all those narrow passages. Pick the 1000 GP out of the chest and walk on, to arrive at the other side of the "second room". Do not go through the passage but go up the stairs. Do not attend the Dark Knight; just go through the passage. Here, avoid the enemies again and keep heading north. You will arrive in the cave-like area again. Kill the Embermen, do not go in the room above, and use the Whip on the stake you have let appear earlier. Step on the switch to reveal an entrance on the wall that had cracks in it. Obviously, that is where you need to go now, so run all the way back there. You can use the Magic Rope now and pick up the previously-ignored Glove's Orb while trudging to the entrance.
In the new area, head all the way north. Step on the switch and flames will appear on your left, making an invisible bridge. Walk on and repeat the process with another switch, and another one. Eventually, you will enter a dark room with a gigantic blob, and a voice will tell you that he, Shade, is going to test you.
After defeating Shade's monster, you will arrive in a room with the next Mana Seed, and an elemental in front of it. Talk to it, and you will learn that it is Shade, the elemental of darkness. In return for defeating his servants, the Sprite will gain its powers. Before you leave, hit the Mana Seed on the pedestal to gain its powers and the possibility to up your magic's level to 5 or 6.