There are many bosses in the game that drop unique items and items needed for Master Skills. The highest of these monsters being the Felankor(Added when the expansion "Reign of Fire Drake" came out). This boss can be found in the dragon cave which can be accessed by getting dragon pearls from the Servants of Isiloon. They spawn in every zone, excluding delos, and special events like wars etc. All bosses drop unique items, different for every boss, and all have a chance to drop high class items (other than maybe kekurikekukaka).