Dark Monk's key
Dark Monk's key is the key required to commence the celebration of Dark Monk. According to the Zeelichian prpohecy, this key should be placed on an altar in Dark Monk's temple at Celebration Island, which would cause Dark Monk's statue to rise out of the volcano.
After Twinsen retrieves all four key fragments, he unites them to obtain Dark Monk's key. Upon completing this quest, he also receives the last level of magic, Fire ball. Following Dark Monk's prophecy and other rumors that he heard, Twinsen travels to the Island of the Celebration. Once there, he enters the Temple of Dark Monk and places the key on an alter that is surrounded by the four statues of the Esmer races. The giant statue of the God then rises up from the volcano.
When Twinsen exits the temple, he encounters Funfrock, who turns out to having been posed as Dark Monk.