Mephistopheles is a sinister character who appears in the Nexus when your Character Tendency reaches pure Black. She will ask you to assassinate other characters, and give you a reward for each kill. When you finish, she will attack you, and upon defeating her you will obtain the Foe's Ring. Since the characters you kill will not respawn until your next playthrough, it is recommended that you do this quest right before the end of the game, and only after you have made sure that you do not need to change your miracles/magics.