The Avengers: The S.H.I.E.L.D. Files

S.H.I.E.L.D. Director Nick Fury has brought together Earth's Mightiest Heroes to form the Avengers. These are top secret S.H.I.E.L.D. files on Captain America, Iron Man, Thor, Hulk, Hawkeye, and Black Widow. But what kind of threat would cause Fury to call six Super Heroes and not just one?