You unlock this stage by finding all 17 Sun Stones available in the standard stages in Old Odyssey.
This area is quite dangerous. As you advance through it, you'll see large pillars spinning in the background. You need to take out enemies and watch for those pillars, which are good at hitting you unless you wait for one spiked bar to pass and then rush after it to give yourself more time. Advance through the area until you come across a Fire type enemy, and gulp it down to gain that ability. You can use it to burn brush. Keep going a short distance beyond that foe and you'll see a door. However, don't enter it. Instead, wait for an overhead bar to pass and then quickly move further right. Destroy some blocks and drop down to grab the Hammer ability from a capsule. Then venture back left and go through the door.
You now are carried gently by a current toward the right of a waterlogged area. You can avoid spiked rings and break apart bomb blocks to proceed. Watch for enemies, which you can hit with the Hammer. When you near a large opening in the background, there's an eel that sometimes swims forward to attack. Below him, in a recess, you can snag a Keychain. Then advance to the right and find a 3D Warpstar to ride into the backdrop. Now you're headed back left, past the eel again (who is now more dangerous) and through more rings. You will eventually reach a door on the far left side of the area, which you should enter.
Through that door, you'll find a ladder to descend. From its base, you can head right through a mirror. Keep an eye on the back wall now as you advance. One of the platforms near the right side of the area is false, so you don't want to fall through it. Past that artificial platform, ascend a short vertical shaft and continue right from the top. You'll want to drop down and walk through an apparent wall, being mindful of the enemy. Then you can float up and grab some meat as you continue right. You can't see it in the foreground, but the floor ahead is lined by spikes, and one of the ledges is artificial. Continue onward while being mindful of those hazards, until you see some stars in a passage below. Drop down through that ledge and head right to find a ledge you can descend to reach a Keychain. Be careful while grabbing it, since the ledge on which it seems to rest is false.
Return to the main ledge overhead and continue right, watching the backdrop. There's a switch you can press here to reach a chest containing a 1up. Additionally, you'll see that you can drop through another ledge, and along that corridor you can pass through a mirror back into reality. Climb a ladder just beyond that point and enter the waiting doorway.
Now you are at the start of an important area. Ahead, there's a lever on the ceiling. You can pull it to trigger an explosion that will cause three keys in the background to drop onto shifting sands that will carry them down along several layers, until they reach a pit at the bottom. You need to advance quickly ahead of them, moving as quickly as you can, so that you can beat them to their destination and use them to run along a ledge, opening three gates. The third gate leads to the Rare Keychain: Boss Butch, which you can only access if you arrive ahead of all three keys. To move quickly enough, you'll have to inhale and swallow the floating circular guys that explode when you get too close. Their ability clears the screen and allows you to not have to slow down overly much.
Once you grab the loot, head through the door to the upper left and you'll appear in a chamber with Clown enemies to either side and a serpent in the background. As you advance right, you'll have to avoid fireballs spewed by such serpents. Then you can catch a 3D Warpstar ride to the background, then advance along another corridor to yet another 3D Warpstar. Next, you'll move right along a passage and notice a large cluster of yellow blocks. Break them apart to reveal a glowing doorway, which you should enter.
Through that door, you'll find a puzzle that requires the Fire ability. If you don't have it, exit and head left or right to snag it from an enemy in either direction. Then return. You're trying to solve another fuse puzzle by lighting a cannon and firing yourself up through the overhead block. A block that slides based on how you move the handheld is available, and you simply need to use the rope fragments to connect the blaze. After you start the fire going, you want to first set the straight rope on fire, which you should then slide into position so that it meets the right side of the rope along the bottom of the screen. Then you want to quickly slide the block left so that the fire meets the lower end of the curved rope. Finally, slide the block back right to connect the top side of the curved rope to the rope that extends down from the cannon where Kirby should by now be positioned. Your reward for completing the challenge is a Sun Stone. If you mess up along the way, you can exit and reenter the room to try again.
Once you've solved the puzzle, exit through the overhead door and proceed right along the corridor. You will find a 3D Warpstar that you can ride into the background. Do so and then proceed right along blocks over a bed of spikes. You can ride a 3D Warpstar to the foreground. There's a door to the right, but you can head left first to grab a Keychain, if you want. Then pass through the door.
On the other side of that glowing door, you'll find yourself in a rapidly flowing stream. It ends just beneath a ledge where you can get the Wing or Circus ability from a capsule. Then you can find a 3D Warpstar just right from there that will take you to a waterfall. Swim down it to another 3D Warpstar that you ride to the foreground, where you can snag an Assist Star and continue right to meet a mini-boss.
