The Squadron Sinister debuted on the last page of The Avengers #69 and were cover featured and the focus of The Avengers #70 as part of the Kang-Grandmaster trilogy which also revived the Golden Age "Invaders" an issue later. There is a ton of creativity in the three issues which comprise this story, a story which made the Black Knight an Avenger.
The Squadron created by Roy Thomas and likely designed by Sal Buscema, were of course evil counterparts to the Justice League of America with Hyperion being Superman, Nighthawk being Batman, Doctor Spectrum being Green Lantern, and Whizzer being the Flash. It was not too hard for even my youthful mind to make that connection, even back then.
This quartet of villains proved quite popular, especially Nighthawk who went on to become a reformed villain and perpetually tragic figure in The Defenders. The Squadron Sinister of course soon inspired the other-dimensional Squadron Supreme a few years later, and that team has become one of Marvel's more interesting conceits, especially in the hands of the late Mark Gruenwald.
Here's a look at the original artwork for this iconic clash cover. The artwork is by the dynamic duo of Sal Buscema and Sam Grainger. This was only Sal's third Avengers story and his fourth Avengers cover. He came to the game well developed and hit them out of the park from the get-go.
This can be seen readily when you grok Sal's first run as the Avengers artist. His first stint was much shorter than memory might suggest, only five issues and six covers total. All the covers are inked by Sam Grainger.