A major mystery in Season 4 of Harley Quinn has been who killed Nightwing. Admittedly, the death initially felt like a joke, but it turns out that Dick Grayson was indeed murdered. Unfortunately, Harley and Batgirl had no clue who did it, as they weren't really feuding with anyone at the time.
Source：https://www.cbr.com/harley-quinn-season-4-nightwing-killer/
