Legion tries to go back in time to kill Magneto, to make up to his father. The X-Men follow him in the past and try to stop him, but they fail. When Legion is just about to kill the Magneto of the past, the Charles Xavier of the past jumps in front of the blast, ending his life. This causes an alternate reality to start, resulting in the Age of Apocalypse.

All credit goes to the original scanners.