The skull, on the bookshelf, has a cool hat.
Talk to the skull until he tells you that he needs a tickle.
Yeah, he needs a good laugh every now and then. Can you tickle him?
Yeah, you would need some item that helps you tickle him.
Check out the impressive desk. ?
Look at the notes on the desk
Pick the feather and the ink from the desk by looking at the notes on the desk.
Use the feather to tickle the skull. You can now change form.