Q)What and where is the Secret Mirror?
A)By "SECRET MIRROR," do you mean the mirror that your trying to recover the pieces to get to the last part?
Well if you do, then it is the mirror that is held up by columns that you are near when you start (meaning when you start in the HUB). When it is gray, you do not have all the mirror pieces. When it is Gold, you have all the mirror pieces. When you beat a boss for the first time and collect the mirror piece, you will see a animated screen where the mirror piece puts itself back on the Amazing Mirror. When you get all the mirror pieces and go through the AMAZING MIRROR, fight really hard Bosses.

Once you get all the mirror pieces,go to Rainbow Route and go into the mirror.