Go to the begining of the game at the beach. When you come onto the beach go left along the wall and you'll see a cave. Go into the cave, and at the end of it there will be a door. Launch either a dark or light spell at the door to open it, and there will be a portal behind the door. Once you go through the portal you'll need to find another portal that will let you get to the next floor. When you get to the fourth floor you will find the Eternal Sword.