This is the standard ending all gamers must have witnessed, mainly because who the heck likes collecting Riddler trophies and puzzles without completing the story first? After apprehending a few of Gotham’s most wanted, you go on to stop the scarecrow and, in the process, you end up revealing to the world that Bruce Wayne is the Batman.
You end up defeating scarecrow and keeping Gotham safe, but now the world knows the real identity of Batman! Reporters accumulate outside the Wayne manor and Batman instructs Alfred to initiate the Knightfall protocol as soon as Batman is done cleaning up the mess. The game doesn’t end there though, you will have to apprehend Gotham’s Most Wanted to initiate the Project Knightfall ending.

If you apprehend 7 of Gotham’s most wanted (either before or after the final cutscene), Batman will take off his cowl and retire as the Batman. He instructs Alfred to initiate Project Knightfall, while sending the Batmobile attached to the Batplane on autopilot to Wayne Manor. Outside the Wayne Manor are reporters waiting to interview Bruce Wayne as he has been exposed to be the Batman! But to everyone’s surprise, the Wayne Manor explodes, by none other than Alfred!
This is the incomplete project Knightfall ending! If you remember from the game; there are 11 suspects to be apprehended in Batman Arkham knight.

By performing all of the extra tasks in the game, you can unlock the secret ending which is also known as the complete project Knightfall ending. If you apprehend all of the suspects and perform all of the side quests in the game, you will finally get the marker for the last and final mission in Batman Arkham Knight. You have to get on top of the GCPD building where the Bat signal is located and initiate project Knightfall just like the last ending. But this time the game doesn’t end with Wayne Manor’s explosion! After Batman takes off his cowl and Alfred blows up the Wayne Manor, we can see that Gordon has been promoted from Commissioner of the GCPD to Mayor.
Furthermore, Barbara Gordon (Batgirl) marries Tim Drake. Finally, we see a family similar to Bruce and his parents leaving the opera when a thief corners them but this time around a mysterious figure jumps on the thief and the fear makes the criminal run away.
Basically, even though Batman has gone, the fear of the Bat still embodies the criminals in Gotham, so his job is finally complete!