During Marvel's "Heroic Age," Steve Rogers became the Earth's next "top cop," much like Nick Fury. Rather than create a new organization, like S.H.I.E.L.D., however, Rogers formed several new Avengers teams. One group in particular, was chosen as a black ops unit: the Secret Avengers. The intention behind the Secret Avengers was to clean up the messes made by Norman Osborn, Rogers' predecessor. This group was intended to act in the shadows, dealing with the murkier threats which the other Avengers couldn't handle. One recurring foe which the Secret Avengers encountered was the "Shadow Council," led by the mysterious Max Fury.
One of The Secret Avengers' most notable battles with the Shadow Council took place in Secret Avengers #16, by Warren Ellis and Jaimie McKelvie. The Secret Avengers had a rotating cast, mixing and matching some of the toughest and smartest heroes in the Marvel Universe. On this particular mission, Rogers chose Black Widow, Beast and Moon Knight to accompany him. Together, these Secret Avengers set out for an underground city called the "Subland Empire." This city was built a mile beneath Cincinnati, Ohio by the Secret Empire. At this point, the city was being repurposed by the Shadow Council. Beast detected the Subland Empire through stray particles of Von Doom radiation, which is only emitted by Doctor Doom's devices.
Realizing the threat of Doom's technology, the Secret Avengers set out to shut down the Subland Empire. Rogers, Black Widow and Beast traversed the city with a nuclear-powered atomic car, while Moon Knight went on aerial recon. Of course, these Avengers came under immediate fire by the Shadow Council. With cars, helicopters and especially guns, the Shadow Council's troops pursued the Secret Avengers through the city.
Delving deeper into the Subland Empire, the Avengers discovered the true purpose of this city. The Subland Empire was outfitted with a giant version of Doctor Doom's old time machine. Using this device, the Shadow Council could transport Cincinnati through time and space, using it as a sort of time-bomb to destroy anywhere on Earth. While the Secret Avengers fought the Shadow Council's troops, Beast sought a solution to this dangerous problem.
Beast figured out how to use the Avengers' nuclear vehicle, turning it into a neutron bomb that would destroy the entire city. Setting the car to detonate, the Secret Avengers hopped onto a helicopter and escaped to the world above. In the aftermath, Beast felt immense guilt towards the number of people he killed. Yet Rogers reassured him that the Secret Avengers had saved countless lives. After this particular adventure, the Secret Avengers went on a few more covert operations, before "Fear Itself." During this event, Bucky Barnes was killed, leading Steve Rogers to become Captain America once again.
As a result, Cap handed the Secret Avengers over to Hawkeye, who shuffled the roster around a bit. This version of the team also took on the Shadow Council, who formed a new Masters of Evil. Defeating these villains, the Secret Avengers then fought a group of robots called the Descendants before promptly disbanding. Another Secret Avengers team was formed as a covert faction S.H.I.E.L.D., led by Maria Hill and Daisy Johnson. These Avengers would have their memories wiped at the end of every mission, due to the sensitive nature of their operations.
Ultimately, while the Secret Avengers have taken many forms, their Subland Empire adventure will forever live in infamy.