There were actually five “shadow” stories, running from Avengers #8 through 12, and written by the great Walt Simonson. The story began with Loki’s goal to merge the “unreality” that he and the Earth’s heroes found themselves in after Onslaught with the universe from whence they originated.
His machinations bring forth the “real” Thor, who (of course) immediately fights with “fake” Thor–despite the fact that they are both heroes and there’s really no rhyme nor reason for fisticuffs. But that’s what heroes do when they first meet. They fight.
This story is where Heroes Reborn starts to reach a conclusion. Lots of fun.