The Light Sword is the most powerful secondary weapon in the game. When in the Mystic Caves, you will find a door that you must unlock via a switch. Immediately after that door, there is a drop in the floor. Drop down and break the left wall. Break the rack to pick up the Light Sword. It is unbreakable, but if you throw it you it will be lost. The Light Sword has maximum damage and maximum attack rate.