One of my favorite Marvel Universe concepts was introduced back in the waning days of the silver age when Roy Thomas debuted a team of villains within the pages of the mighty Avengers that were villainous pastiches of the DC Comics super-group, the Justice League of America.
The Squadron Sinister were created by Thomas and first appeared in Avengers #70 (Nov. 1969). They were assembled by the cosmic entity known as the Grandmaster to act as pawns in battle with champions chosen by the time-traveling Kang the Conqueror; the superheroic team the Avengers. The Grandmaster created four villains (Nighthawk, Doctor Spectrum, Hyperion, and the Whizzer), to face Captain America, Iron Man, Thor, and Goliath. The four then battled the Avengers, but all were defeated and discarded by the Grandmaster after he left Earth.
The Squadron Sinister was reunited several years later, when Hyperion encountered the alien Nebulon. Hyperion had been miniaturized and imprisoned in a glass sphere by Thor in their earlier conflict. Hyperion promised the alien the planet Earth in exchange for his freedom. The Squadron then created a giant laser cannon in the Arctic and planned to melt the polar ice caps, thereby covering the Earth's surface in water. Only Nighthawk had reservations and after trying in vain to contact the Avengers, (Nebulon had rendered the Squadron Sinister invisible and intangible while in the presence of the Avengers), he located the Defenders, who agreed to help Nighthawk and they all teleported back to the Arctic.
Comprised of Doctor Strange, the Hulk, Sub-Mariner, and Valkyrie, the Defenders battled against the Squadron and defeated them. Nebulon was overwhelmed by sheer force of numbers, and at the last moment reverted into his true alien form and teleported away with Hyperion, Dr. Spectrum, and the Whizzer. Nighthawk opted to remain behind with the Defenders and ultimately joined the team. One year later, Nebulon returned the Squadron Sinister to Earth in possession of a new energy-draining weapon, with which the Squadron Sinister planned once again to conquer the world and they also found themselves battling the Defenders yet again. Despite an initial victory, the Squadron was soon defeated by the Defenders and the Avenger Yellowjacket.
Several years later, the disbanded Squadron members were contacted individually by the Avengers, who were seeking assistance in freeing their fellow member the Wasp, who had become bonded to the original Dr. Spectrum's Power Prism. Billy Roberts (the second Dr. Spectrum) tricked the Avengers and reclaimed the Prism, but he was defeated in battle.
The team then vanished into obscurity for several years. During this time, Hyperion battled Thor and the Thing, and traveled with the female warrior Thundra to the alternate-universe Earth of the heroic Squadron Supreme, where he impersonated that teams version of Hyperion for several weeks before he eventually died in battle. The Whizzer returned to a life of crime, but wishing to cut his affiliation with the Squadron Sinister, he adopted the alias of Speed Demon along with a new costume. Speed Demon briefly joined the super-villain team the Sinister Syndicate, before reforming and being recruited to join the superhero team the Thunderbolts.
Soon after this the Grandmaster reappeared and reunited the Squadron Sinister with an apparently resurrected Hyperion and an all-new Dr. Spectrum (Alice Nugent, a former lab assistant of Henry Pym). The group tried to coerce Speed Demon and Nighthawk into joining, but both were initially reluctant. Ironically, the New Thunderbolts themselves were responsible for the pair rejoining the Squadron Sinister. Speed Demon was ejected from the New Thunderbolts for committing a series of robberies, while Nighthawk (who had recently begun working with the team) quit in protest after Baron Zemo rejoined and he learned that he was being used simply to finance their activities. Courtesy of a phenomenon known as the Wellspring of Power - an inter-dimensional source of superhuman abilities - the Grandmaster increased the Squadron Sinister's powers and they battled against the New Thunderbolts, as Baron Zemo also wished to control the Wellspring. Zemo managed to defeat the Grandmaster, but in the ensuing chaos the Squadron Sinister scattered and escaped.