After heading down the rope you'll encounter another new enemy: a knife lizard. These things jump very high and will throw out a couple of knifes at you. Try to get close and whip the knife lizard as it lands from jumping. Move down the rope and you'll find an upside down punaguchi waiting, try and time your attack so you take out its projectile as soon as it spits one out. Watch out for the water in this area -there are lots of fishmen hiding here, and they will shoot off projectiles at you. You'll have to go over a bottomless pit by using three breakable platforms, make sure not to wait around on them.

Move across the next water pool, killing the fishmen along the way. There is one last breakable platform in this area which you'll have to jump on the reach the rope at the far left. Move up this rope to enter a small room with another new enemy: octos. These jellyfish-like enemies will bob up and down and in addition to damaging you upon character, they'll also steal some hearts. Kill them and then head up the rope at the right, this will lead to another room where the alternate routes realign. You'll find a single punaguchi here, kill it and then head through the door.

Head left and use the breakable platforms to get over the bottomless pit safely. Continue on and you'll arrive in a small arena with two pillars in it, whip the candle and collect the crystal to make the Lord of the Crystal Castle to show himself: Dark Side. A large cloud will form in the sky and then a crystal will appear. Immediately start attacking this crystal as it's the only way to damage the boss.

After a few seconds the crystal will change into a cloaked figure who can't be harmed. He'll wait a few seconds and then raise this staff, causing the cloud to send down a lightning bolt. This bolt will strike the ground and cause some energy waves to erupt either side of it, so move out of the way before they hit you. Dark Side will then change back to his crystal form, at which point you need to resume attacking him. Repeat this process until Dark Side explodes.