Objective: Initiate the Knightfall Protocol
Once you have finished the main storyline and completed at least seven of the ‘Most Wanted’ missions, you are able to activate the ‘Knightfall Protocol’ from the mission select screen. This will start the epilogue.
To complete the mission we simply need to head to the top of the GCPD HQ building, approach and interact with the Bat Signal here. This will trigger a cut-scene and credits to end the game. It should be noted that although you can complete the game with seven ‘Most Wanted’ missions completed, this will also give you a somewhat truncated ending.
Note: For the true, complete ending you’ll need to find and complete all fourteen ‘Most Wanted’ missions before completing this objective. Fortunately, you can do it as many times as you like and return to free roam afterwards!