Who's This? The Tyrant of Apokalips.
The facts: Without a doubt the most popular/important/ubiquitous character in Jack Kirby's Fourth World, Darkseid made his first cameo appearance in Superman's Pal Jimmy Olsen #134 (December 1970) before making his first full appearance in Forever People. The fascist leader of the evil New Gods, he would only infrequently cross paths with non-Fourth World characters in the pre-Crisis era - even the popular Great Darkness Saga uses him as a background threat - but with the first crossover event of the post-Crisis DCU, Legends, it seems all bets are off. He has since become the most overused of villains, the biggest of the bad, not only appearing in many New Gods-related projects, but more big events like Final Crisis and the birth of the New52's Justice League.
How you could have heard of him: For comics readers, he's hard to miss, but for others, I believe he owes his popularity to animation. For a certain generation, he will be remembered as the main villain of the Super Powers cartoon (there's a great action figure of him to go with it), and later, he'll be an important part of the Superman Animated Series and the Justice League follow-ups. Most of the cartoon series have used him, including Teen Titans Go! where he is voiced by Weird Al Yankovic(!). He appears in the Justice League movie too, but only in the Snyder Cut, so I don't care.
Example story: New Gods #7 (Feb-March 1972) "The Pact!" by Jack Kirby and Mike Royer
By then, Heggra is dead and Darkseid is sitting on the throne. His wife Tigra is outraged that their son would become part of this "solution".
Darkseid the cold manipulator is much more fearsome than the big brute who physically tangles with Superman and the like that we get later, no?
Who's Next? A tattooed mercenary.