Haunted Castle

Haunted Castle is the Third Sea island for levels 1975 to 2075. It is a ship-like island, the third-largest in the game, with an enormous mast that makes it the tallest map. You find it by sailing to the Castle on the Sea and looking for a flag surrounded by giant walls. Two quest givers run the grind. Haunted Castle Quest Giver 1 asks for 8 Reborn Skeletons at Lv. 1975 (13,000 Beli and 91,000,000 XP) and 8 Living Zombies at Lv. 2000 (13,250 Beli and 93,500,000 XP). Quest Giver 2 follows with 8 Demonic Souls at Lv. 2025 and 8 Possessed Mummies at Lv. 2050, paying up to 13,750 Beli and 98,500,000 XP. Ghosts also roam the island. In the graveyard you can pray at a Gravestone for random rewards, and inside the mausoleum the Death King exchanges Bones for a Random Surprise, limited to 10 rolls per 2 hours. An altar with blue flames on the way to the cemetery summons the Soul Reaper raid boss when you place a Hallow Essence on it. There are 13 chests in total: 5 Silver, 5 Golden and 3 Diamond in the crow's nest. The wiki also lists it as the place where the Skull Guitar puzzle begins and ends.