In the movie X-Men: Apocalypse, the same is the king among the mutants, the most powerful of all that exist. From the beginning of the times Apocalypse has always been different, because all his companions have worshiped him as a God, which made his power grow and grow, because he fed on the power of other mutants.
After thousands of years asleep he has awakened and returned to life. The world he sees is very different from the last one he saw before sinking into a dream that has lasted thousands of years, so he makes a very drastic decision.
Instead of changing the world he does not like, he decides that the best thing he can do is to destroy it and start a new world, one that he likes more. For this he recruits Magneto, who will be in charge of looking for mutants that agree with this new world.
For his part, Professor X does not agree with these plans, so he is in charge of recruiting a group of mutants to fight, a group of young people without too much experience that is going to have very complicated against Apocalypse and his Henchmen.