Aunt May got shot by one of Kingpin's men (they were aiming for Peter and screwed up); so instead of doing the logical thing and get one of the many capable healers in the Marvel U, Peter Parker makes a deal with Mephisto (remember that devil guy from the Ghost Rider movie? Yeah, him). The deal: Aunt May survives alive and well, but Peter's marriage to MJ and all memory of it vanish from the timeline. Harry Osborn is back alive, apparently having lived in Europe for all the years since his death. Nobody remembers Spiderman's identity, but some things, such as his tenure with the Avengers, are still canon... somehow.
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