Some dungeons may be paired with a skylands island. Skylands are an island-based world containing dangerous custom mobs which can drop exclusive materials. You can access these worlds by navigating to the /dungeons menu and clicking the world.
Dungeons are objective-based timed raids that contain mobs, loot chests, and bosses. Upon defeating the final boss of a dungeon, you can claim 1 of 3 reward chests which contain Runes, Custom Enchantments, Essence Powder, Materials, and Custom Items.