Marvel's greatest heroes sacrificed themselves to save the world from the deadliest threat they had ever faced: Onslaught! The lives of the Fantastic Four and the Avengers were saved only when Franklin Richards created a parallel universe, where we found those heroes, reborn! But this world has long since merged back into our own, and the evil Onslaught -- forged from the most vile parts of Magneto's and Professor X's psyches -- has been presumed long dead. But now, he, himself, is reborn -- and he's bringing fragments of the other universe with him! Join comics superstars Jeph Loeb and Rob Liefeld in this no-holds-barred, all-out action extravaganza -- celebrating the 10th anniversary of the event that changed Marvel history -- as Onslaught, the Marvel Universe and the Heroes Reborn Universe collide in this massive crossover!
Obviously, this is one of those instances where the wiki entry is a simple plot synopsis (which is too bad, because I hate Marvel's advertising copy). If you like spoilers, get More Info here.