Injustice: Gods Among Us game and comic spoilers ahead! Don't say I didn't warn you!

Damian Wayne, the son of Bruce, had killed Nightwing by accident. Batman responded with an anger unmatched, which started down Robin's slippery slope to joining Superman's cause. By the time the game takes place, he's effectively become the second Nightwing, working for Kal-El's team.

I used an Avengers Assemble All-Star Loki, repainted and with the Injustice Nightwing's escrima sticks.