Who's This? The green-clad guy with the goofy teeth at the bottom of page 2 in Who's Who vol. XX.
How you could have heard of him: Sabbac was reintroduced in 2004's Outsiders #7, where Judd Winick revealed him to be Freddy "Cap Jr." Freeman's foster brother - his origin told in the Superman/Shazam! mini the next year - but by the next issue was replaced by a new, more demonic-looking Sabbac, Ishmael Gregor, who went on to be featured later in the series and in 52.
Example story: Captain Marvel Jr. #6 (1943) by Otto Binder and (probably) Al Carreno
1. Satan (HIMSELF!) is basically a guy in a Halloween devil costume.
2. There's a demon called "Any". I've never come across the name in my demonology studies, so I'm guessing this is a rotating spot for, literally, ANY demon who wants to give Sabbac a little juice.
3. They're not keen on helping Sabbac because they're scared shitless of Captain Marvel Jr.! Hilarious!
And 4. The lightning climbs UP from Hell to strike Sabbac and give him his powers! The demons actually travel along the lightning to "possess" Sabbac.
Junior fails to foil his escape because Sabbac can burrow through rock and visit Hell personally to have conversations with his dark princes. They remind him that it's not use being out of jail if Captain Marvel Jr.'s gonna put him right back in again. What he needs is a plan. And his trusty book of black magic suggests one!
I wonder why Sabbac didn't return more often, though I guess it was relatively rare for villains to recur during the Golden Age. As one of the reverse-Marvels (he's pretty much Junior's version of Black Adam), he should have had more play. My working theory was that maybe by the time it would have made sense to use him, the Comics Code was too strict for a demon worshiper. Except Captain Marvel and friends stopped publication before the Code, and started up again in 1973, when the Code was definitely more relaxed. I guess he just fell between the cracks.
Who else? Haven't done a historical hero in a while, have I? Maybe I should.