Empire ‘s March edition has assembled The Earth’s Mightiest Heroes onto their cover. Their cover stars headline the most comprehensive Avengers: Age Of Ultron coverage to be found anywhere in the nine realms (well, the Midgard part of them, anyway). See all the images after the Jump... Marvel Studios presents “Avengers: Age of Ultron,” the epic follow-up to the biggest Super Hero movie of all time. When Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry and Earth’s Mightiest Heroes, including Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow and Hawkeye, are put to the ultimate test as the fate of the planet hangs in the balance. As the villainous Ultron emerges, it is up to The Avengers to stop him from enacting his terrible plans, and soon uneasy alliances and unexpected action pave the way for an epic and unique global adventure. Marvel’s “Avengers: Age of Ultron” stars Robert Downey Jr., who returns as Iron Man, along with Chr