Shield, does not destroy the Marvel Universe that you know and love… at least not in the first issue. It does lay down the framework for one of the largest retcons (retroactive continuity) in Marvel’s history. Shield’s history spans throughout the ages, and begins with the first Brood attack on earth. The Brood is an alien race that takes over their host body in order to infect and colonize the planet of their choosing. However, they are defeated by Imhotep, an Egyptian warrior holding a spear and a shield.
Jonathan Hickman (Nightly News, Red Mass for Mars) tells the story of Leonid, a teenage boy that is taken to the Immortal City, an amazing sub-city underneath Rome. He is taken before the high council of the city and the order of Shield is explained to him. Hickman cherry picks great historical figures as agents of Shield to give the overall impression that they are everywhere. In one such case Galileo creates a device to stop Galactus from eating earth years before Reed Richards was ever forced to do so.