Kuraigana Island

Kuraigana Island is a dark and gloomy place in the Grand Line. It was once the home of the now fallen Shikkeāru Kingdom (シッケアール王国 Shikkeāru Oukoku, Muggy Kingdom in the Viz Manga and FUNimation subs). The forest island is relatively deserted except for some ruins. Amongst its landscape, the island possesses unique hills. These hills resemble the Curly Hill in the animated film The Nightmare Before Christmas. The name of the island, Kurai, means dark in Japanese. This island is also the residence of Dracule Mihawk.

History
Seven years before the current storyline, a terrible war was fought on this island. The baboons inhabiting the forest, gifted with the capability of learning from humans, watched the conflict, learned how to fight using weapons, and became fearsome fighters. At some point after the war's end, Dracule Mihawk arrived on this island and made the castle his home.

Perona was sent flying here by Kuma's Nikyu Nikyu powers when she told him where she wanted to go if she went on a trip. Upon arrival, she settled in one of the ruins. Roronoa Zoro was later sent to this island by the same Shichibukai. The two met up with Dracule Mihawk as he returned from the Battle of Marineford. Mihawk told Perona about Gecko Moria's presumed death, and gave Zoro a little boat to return to his crew mates, later telling him about the island's history and warning him about the baboons' threat. This is where Zoro trained with Mihawk for two years.

Anime and Manga Differences
Originally in the manga, the Humandrills were simply hinted at with a silhouette in Zoro's cover story. In the anime, they were given a slightly more detailed foreshadowing with them attacking Zoro in the ruins as ape-like silhouettes.

Trivia

 * "Kuraigana" means "It's really dark".