After heading up the rope you'll arrive in a small room with two vampire bats in it, kill them both and then head up the rope to the right. This will lead to a small passage with an upside down punaguchi waiting, try and time your attack so you take out its projectile as soon as it spits one out. After killing this enemy, head left and you'll come across a lot of breakable platforms and more vampire bats.

The bats can get in the way of your jumps, so try taking them out as quickly as possible. Once you see a rope, head down it and you'll 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 down 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.