X-Men (vol. 2) #41
Title: “Dreams Die!”
Medium: comic
Cover date: Feb. 1995
Publisher:
48 characters in this story:
In this issue, the powerful mutant Legion goes back in time and attempts to kill Magneto. But Charles Xavier (incidentally, Legion's own father) intervened and sacrificed himself to save Magneto, his friend.
This caused an alteration in the timeline that resulted in the present being ruled by the powerful mutant despot Apocalypse. The alternative timeline was known as a the "Age of Apocalypse" and was central to a massive crossover event that incorporated all of Marvel's continuing X-Men titles. The various series were actually suspended for four months under their regular titles. Each series continued with issues one through four under a different title, focusing on their characters in the alternative timeline.
X-Men (volume 2) #41 continued the next month in Amazing X-Men #1. After four issues of Amazing X-Men, the regular timeline was finally restored and X-Men volume 2 continued with issue #42, with a cover date of July 1995.