Who's This? He's on page 2 of Who's Who vol.XVII.
How you could have heard of him: Uhm... in Nocturna biographies? While his more famous (adopted) sister has made appearances in the post-Crisis and even New52 universes, Night-Slayer might not even have existed in those realities.
Example story: Detective Comics #529 (1983) by Doug Moench, Gene Colan and Dick Giordano
Who else? I think Nocturna got more than a fair shake in all this, so she won't be the subject of this feature for now. Looking ahead at through Volume 17 of Who's Who, I still think it'll have to be a darkness-related character I cover next.