Chekhov's Gun: The key to raising the Ancient City is the Gold Pedestal you sold to Pim in the first game.
Grail in the Garbage: The Gold Pedestal that Adol sold to Pim's Trading Shop in the first game is the key to raise the Ancient City.
Hopeless Boss Fight: The first and third phases of the Final Boss in Mask of the Sun. After dodging Eldeel's lightning attacks for a minute, Adol will solve the Moon Logic Puzzle and figure that you have to use the MacGuffin necklace on him. During the third phase, after dodging him again for a while, the Disciples will destroy his armor with a group attack.