Ys: The Ark Of Napishtim Cheats

At the main menu of the title screen, highlight "Option". Hold "Circle" and "R", and then press "Right" on the D-Pad. A secret menu with garbage (untranslated) text will appear. This is a list of all of the in-game movies; press "X" to view a movie. Select the last option, or press "Circle" to close the menu.

Simply beat the game on any difficulty and Nightmare Mode will be unlocked.