So Damian is fighting Not-Joker, while still trying to remain true to his father's wishes and not killing anyone, no matter how close he comes to pulling that metaphorical trigger... then he finally gets to Not-Joker, who reminds him of the standard trope of Joker always comes back because Batman never has the guts... So Batman rips out Not-Joker's guts, code be damned! Remember that time he did that to Michael Lane? Pretty much the same thing. Damian apologizes to his father, picks him up, and then leaves.
But what's this! Not-Joker isn't dead... yet... because a single, silenced bullet pierces his forehead, and looky there, the real Joker is back... Yay?
So the real Joker is back, and apparently up to no good again, even though we never see this... What we do see is Alfred the cat, sitting at the computer, telling Damian about all this...
So Damian finally accepts his role as Batman and goes out and does Batman stuff or something, looking for who else was behind Dick Grayson's death and the real Joker... I guess.