This is achieved after you have completed the main storyline and then every single sidequest, that includes all 243 riddles and apprehending the Riddler.
Once you have locked up all major threats in Gotham you will be able to unlock Knightfall Protocol. The waypoint will direct you to the spotlight with the bat symbol atop the GCPD building and ask you to press
to initiate.
Doing this will show you the full ending of the game and then unlock the Knightfall trophy for you.