Items: Mystery Bag, Old Scroll, Shortcake
Though by the time you can reach Ruby Cave you can also head along Topaz Path (which is actually closer to Monster Village), you should start with Ruby Cave. It's an area that was visible on the map a while ago, but you couldn't get there until now. Since that has changed, it's time to do some exploring.
From the entrance of the area, head up a staircase. You'll see a frozen fountain at the center of that area. Beyond it, there's a statue that won't let you through the gate unless you satisfy his criteria. He gives you a clue: "One of my brothers differs from the rest of us. Where does he reside?" You can pay 1000 coins for a clue or you can pay 5000 to pass. Or, you can answer the question correctly. The answer has something to do with Spinach Caverns, though I don't know what. I just pay the 5000 to pass, since money is common by this stage in the game.
Beyond the statue you'll head up some stairs and into the next room. There, you're at the start of a path leading forward toward three vases. Advance to them, break them, then defeat some spidery enemies beyond. They fall easily to Spin Drill attacks, or even to a flurry of normal strikes. Beyond them, drill a stone wall with Ice Parts equipped to reveal the next corridor. Head down it and as you walk into the next area, you'll fight more of the spidery creatures and an armored Phantom. All should fall easily to short drill attacks. Continue along the path and you'll be able to break two vases. There's a semi-transparent guy hiding in the lava here, so make sure you defeat him before continuing left along the path. Ahead, a Phantom will drop into a spiked ball and fall to an area below. You'll also see some ledges you can't reach from where you stand. It's time to drop down in the direction the Phantom went.
When you land, be ready for that Phantom that you saw a moment ago. He's got a fairly long life meter, so expect a tough fight as such encounters go. Wear headgear that can protect you from trap hazards, too. The level-5 nightcap is perfect. While you're taking care of enemies, watch out for two of the semi-transparent guys hiding in the ice pools around you. They're good at hiding and screwing your chances at perfect level completion. When the enemies are gone, head to the base of a ledge with a ladder and a weak wall you can drill to its right. You'll find a cache of coins. Now climb up the ladder and up top, there are two Phantom enemies. Defeat them quickly and you'll notice two treasure chests on higher ledges. One contains SHORTCAKE, the other a MYSTERY BAG. They're rather difficult to reach, but if you run and jump at the walls then twist sideways as you start to fall, you can grab the ledges and pull yourself up to open the chests. There's a gate beyond those ledges to the next area, but you can't open it yet.
Turn back around the way you came and you'll see a narrow ledge extending out toward a beige-colored platform. Hop to that ledge and step onto the platform to cause another to appear. The platforms here disappear much more quickly than any you've encountered before. Run directly forward and plenty will appear as you go, so that you'll reach the far ledge. There, break open some pots and refill your energy on the charging station, if desired. Note that one of the pots in this area--next to the white tile with red markings--is actually a Phantom in disguise. Beat him, then step on the white tile to make some boxes appear down on the lower level of the stage. Head back over to the energy refilling station, then start across the left beige platform. You'll want to run in a straight line until you're even with the red crystal to your left. You can stand there on a more solid ledge that appears and drill the crystal to send the column above it falling to the left. Now cross over that column to the ledge.
There's a platform there you can jump onto, with a junk bin resting on top. Break it open to collect some junk, then drop down behind it and step on a circular yellow switch. Parin will warp through to a new area. Continue along that hallway, while watching for spikes. One of the semi-transparent goons will also attack, so beat him. At the end are the two crates you dropped. Push them off the end of the ledge and then drop down to the area below. Now push the column of two crates over to the switch. Once they are put in place, the crates will keep the switch pressed and open the door at the edge of the corridor so you can pass through to the next room.
You're at the start of another hallway. Continue along the hallway toward the area beyond and there will be electric lights sparkling across the area, with Phantoms held in them. Equip something to avoid trap damage, like the high-level nightcap, and dash guard through, then turn around and destroy the poles that are generating the electricity. Take out the two Phantom enemies, as well. Beyond there, spiked holes will allow spikes to come up from underneath. Walk around there and you'll see a stack of crates and also a raised ledge with a treasure chest. I haven't figured out how to reach the dang chest yet. Sorry about that. Nearby, there's a weak wall you can drill to enter a new room. Do so.
As you enter this room, an armored Phantom will attack. Drill him quickly, as he has lots of defense and also carries a powerful weapon. When he's gone, you're free to explore in safety. There are several iced over pools in this room, and weak walls to the left and back sides of the room. The back wall holds a coin cache, the left a ledge with three levers. They have to be pulled in a certain order, I believe middle, right, then left (though that could be wrong; someone correct me if it is, please) or you won't get them all facing away from the entrance at the same time (which is your goal). When you pull them correctly, you'll lower a nearby barrier. Pass through and there will be four Phantoms waiting to wail on you. Drilled shots are necessary here, and these guys move quickly and work as a team in the narrow corridor. Keep restorative items in mind, as you may need them.
Once you destroy those Phantoms, you can continue left (destroy the semi-transparent guy that lurks here in the shallow ice area) and there are some spiked balls floating around, plus more Phantoms just short of the doorway into the level's next area. The doorway is blocked by a boulder. Kill the Phantoms, drill through the stone (you'll want to have the Ice Parts equipped) and also catch the semi-transparent monster nearby before you head through the door and into the treasure room. Jump to the altar to grab the OLD SCROLL. Now when you return to Monster Village, give it to Rocko. He'll make more mist disappear from the map. Next go to Tiese Town and Pamela will open your mystery bag to reveal CHOCOLATE. Now head to the world map and make your way to the next dungeon.