The island is where the legendary Shichiseiken, a cursed blade, was sealed. Every one hundred years, there is a red full moon that increases the sword's power. Several hundred years ago, three princes fought over the love of a priestess which led to a great war, which is what cursed the sword in the first place.
The sword forced the peoples' rage to increase until the priestess gave her life to seal the sword. The island is also the location of a Marine dojo, and there are three towers connected by a cave that are used to seal the sword's dark power. The sword was destroyed by the Straw Hat Pirates.