Legion Quest
1995David Haller — Legion, the omega-level son of Charles Xavier — travels back in time to kill Magneto before he can become a threat, believing this will fulfill his father's dream. The X-Men pursue him into the past but arrive too late: Legion accidentally kills Xavier instead, creating a paradox that shatters reality and gives birth to the Age of Apocalypse. The prologue to one of the greatest X-Men events ever told.
4 core issues, 2 optional tie-ins/crossovers
Legion Quest part 1 — Legion's plan to travel back in time to kill Magneto is revealed
Legion Quest part 2 — The X-Men discover Legion's plan and pursue him into the timestream
Legion Quest part 3 — The X-Men arrive in the past and race to stop Legion
Legion Quest part 4 — Legion kills Xavier instead of Magneto; reality shatters; the Age of Apocalypse begins
Tie-InX-Factor's perspective on the time-travel crisis
Tie-InCable senses the temporal shockwave; the timeline begins to unravel