One of the most powerful villains the Marvel Universe has ever seen changes everything for the Children of the Atom and Earth's Mightiest Heroes! Behold the psionic force of nature known as Onslaught - but what dreaded foe could terrify the unstoppable Juggernaut? As Cable battles Post, herald of the oncoming doom, Nate Grey faces a ghost from his past- and Onslaught gathers his forces! The Avengers join the X-Men to work out a battle plan - but could the answers lie with finding Magneto? And Charles Xavier has a troubling proposal for the Fantastic Four. But when Onslaught's identity is revealed, who will be the traitor to the cause?