Nightwing
Although Nightwing is commonly associated with Batman, the title and concept have origins in classic Superman stories. The original Nightwing in DC Comics was an identity assumed by alien superhero Superman when stranded on the Kryptonian city of Kandor with his pal Jimmy Olsen. Drawing inspiration from Batman and Robin, the two protect Kandor as the superheroes Nightwing and Flamebird. Following the Crisis on Infinite Earths continuity reboot in 1985, Nightwing was re-imagined as a legendary vigilante from Krypton whose story inspires Dick Grayson's choice of name when he leaves behind his Robin identity.
Nightwing
When the Joker is told he is dying by his doctor, he unleashes Joker juice on the inmates at the Slab, causing a breakout. At the end of the arc, Nightwing is responsible for killing the Joker against the wishes of Batman and Oracle. Nightwing becomes depressed and Oracle tries to bring him out of it. Soon after, Batman manages to revive the Joker.