After fighting Xemnas for the first time all the organisation headstones in proof of existence light up like Saix and Luxord allowing you to fight each member again. Then, once you've beaten all members the next time you go to the door of Kingdom hearts you will have two options.
Face Xemnas
Dive into the Heart
Selecting the second option will take you to the cross roads in the secret ending in the Keyblade graveyard. You can travel down three of the four pathway, when you reach the end a command button will appear showing ???
Select it at each end of the three roads and you will be able to able to fight a knight from the ending. Each vitory will also give you secret report by Xemnas .Once all knights are beaten go down the final path were three keyblades will be show lying in the ground.
You can now choose to have either...
Road to dawn
Mickey's Kingdom Key
nameless blade (keyblade shown with the first knight)
(yes I'm aware I have to much time on my hands)