The result is what you see before here – Hyrule's Most Wanted ... Trust me, it's worth it. Oh Agahnim, you simple-minded fool of a dark wizard you! I could just kiss your face!

You can only get this easy-to-get Heart Piece after you've defeated Agahnim in Hyrule Castle. You must travel northwest of the Sanctuary in the Light World, past the entrance to the Dark World.