CONTAINS SPOILERS

After defeating Astel, Naturalborn of the Void (located by entering the coffin at the end of the blood stream at Grand Cloister, Lake of Rot) a seal is locked that reads: You cannot proceed without a ring of oath. The Dark Moon Ring is found in a chest at the Academy of Raya Lucaria behind Rennala. To unlock this chest you must have completed Ranni's side quest to get the Discarded Palace Key.