Night-Slayer is Anton Knight, a Gotham City criminal and enemy of Batman. He is the adoptive brother and first partner in crime of Nocturna, whom he had fallen in love with. Originally Anton used the alias of the Thief-of-Night to become a thief after his father's death, with the sole purpose of providing a fine life for his adoptive sister. The New 52 reboot would go on to re-introduced Anton as Night-Thief the now stepson and secret lover of Nocturna. His alignment with her made him an enemy of Batwoman.
Night-Slayer was created by Doug Moench and Gene Colan, first appearing in Detective Comics #529. (1983)
