2 of marvels biggest and oldest teams face off over one major force coming to Earth: The Phoenix Force!   The last time it was on Earth Jean Grey almost couldn't contain it and one planet was destroyed in the process.   The Avengers do not want that next planet to be Earth.  The X-Men see the Phoenix Force as a possible salvation for rebuilding the mutant population after the devastating aftermath of House of M.   They clash almost instantly in this opening part to a major event that shook both teams for years to come!