In the Light World. go to the smithy's house. Use the hammer to knock down the post. Then, jump into the hole. Go into the top cave and you will see a red jar. Put some Magic Powder on it. A bat will appear and says that it will curse you. After you do this, any item that uses magic will only take half of what it used to. However, when you do this every time you get hit and it will hurt double.