Higemaru Makaijima/Curse Island

Pirate Ship
You will find the Pirate ship with the key to the Curse Island gate off the west coast of Jack Knife Island, the northernmost island which lies left of center. This ship will be just like the one that you tackled to get the Cuck Island key from, including the number of pirates you are required to defeat: 15. Once you obtain the key from the captain, sail south to Mermaid Island, then sail down the east coast of the island until it ends. Then sail east and you should hit Curse Island. The entrance is on the southern coast.

Curse Island
You start off on the southern end of the island. All to the north of you is a giant cavern in which a strange religous cult has taken residence. Just outside the entrance to the cavern is a large monolith with more incomprehensible writing on it. The majority of the blue hood apostles carry one variety of weapon or another. Some throw knives at you which travel across the screen like the Yhary's spears. Others toss lit candles at you which do not travel far, but burst with an explosion on the ground. Curse Island is not quite as large as Cuck Island, and it won't take you very long before you find yourself in the chamber of the boss in the north region of the island. See the section below for more information about the boss fight.

Interestingly, Curse Island is another island upon which you do not seem to earn anything for defeating the boss other than the ability to advance to the next island. However, there are two mysetiously hidden items on this island, one of which is incredibly difficult to find without learning the proper method in advance. The easier item to find of the two is one of the three hidden crystals which helps you obtain the best ending if you collect all three. Open the spoilers below the boss section to learn the proper methods to obtain them.

Boss fight
The boss of Curse Island is known only as the Archbishop. He will only ever appear in one of two places: to the left of the large statue in the top center of the room, or to the right. When he appears, he summons a good number of fireballs to throw at you while you attempt to hit him with the rocks that are located in the lower corners. You can safely through the rocks through the fire in order to hit him. The Archbishop does not travel between these two locations, but rather teleports. When he is in the process of teleporting, he cannot be hit and damaged. You will have to wait for him to fully materialize in one of the two locations in order to hit him. He will go down quite easily with enough hits.