Nightwing is a fictional DC character who was the original Robin.
Dick Grayson already lost one family. Losing another family drove him to madness. Aquaman came to him and offered a chance to resist Darkseid's rule and end him once and for all. Nightwing didn't refuse the offer.
Nightwing appears as a playable character in this crossover game between Lego Batman (hence the title) and Lego Marvel. He appears defeing the Batcave when it is besieged by Hydra agents led by Carnage. His character token is found in the level (aptly named Assault on the Batcave).