I found it by chance but I retraced my steps and tried this method twice and it seemed to work. After you exit pixies lair, go up to the stairs that will take you back into the graveyard. I then went: Left, Down, Right and you will come to the mirror! its straight past there. If this is confusing, if youve found the shadow hands bit just go down and right and you should be there. I really hope this works for you! if not, keep exploring youll find it eventually like i did :)