X-Men 41-A (1991 1st Series)

to kill Magneto before he can turn on Xavier comes to fruition; With the killing stroke, Xavier sacrifices himself to save his friend; In the present, the X-Men wait as the world as they know it ends; Thus begins the Age of Apocalypse; Continued from Cable (1993 series) #20; Continued in X-Men Alpha #1; Next issue is Amazing X-Men #1.