Change has always been a bit part of the X-Men. Be it roster changes, secondary mutations, deaths or resurrections, or so much more, the X-Men are always in flux somehow, which is fitting- mutation is all about change.
The last 10 years have definitely been tumultuous for the team and that's putting it lightly. They started decade as an embattled minority, with barely 300 mutants around, and have ended it as an emergent world power. In between, there has been all sort of craziness, from unexpected returns to potential extinction level events. This list is going to highlight the biggest changes to Marvel's Merry Mutants this decade.
10 Schism
Wolverine and Cyclops have always had a contentious relationship, but after M-Day the two men put aside their differences and worked together for the good of the remaining mutants. Cyclops was the brains and Wolverine was the brawn. Cyclops put Wolverine's skills to good use.
However, when the new Hellfire Club took out a bunch of the X-Men and held humans hostage, Cyclops was forced to order one of the junior X-Men, Oya, to take out the Hellfire Club's soldiers. This infuriated Wolverine, who was on his way to deal with the situation. Cyclops and Wolverine would come to blows and the remaining mutant community would choose sides, with some going with Wolverine back to Westchester to begin a new school and the rest staying on Utopia with Cyclops.
9 A New X-Force
X-Force, in one incarnation or another, has been around for a long time. Usually, the team isn't exactly affiliated with the X-Men, but more of a covert group that worked in the shadows. All of that changed on Utopia. Cyclops was looking at the very real possibility of the extinction of the mutant race and needed a way to take care of some threats with extreme prejudice.
He turned to Wolverine, a man who has no problem with killing, and had him assemble a team of like minded individuals who would take threats to mutantkind before they got terminal. The existence of the team was kept a secret from most of the X-Men and X-Force did a lot of bloody, dirty work.
8 The Return Of The Original Five
Due to the events of Avengers Vs X-Men, Beast had lost faith in a lot of things. On top of all that, his body was mutating again, a change that could be deadly to him. So, he came up with a radical solution- use a time machine to go back in the time and bring the original five X-Men to the present.
These younger versions of Cyclops, Marvel Girl, Beast, Iceman, and Angel would have a lot of adventures in the present, learning about the world that was awaiting them. They would be a big part of the X-Men books for most of the decade, until 2018's Extermination sent them back to past.
7 Avengers Vs X-Men
The return of the Phoenix Force would put Marvel's premiere superteams at odds. Cyclops believed that the Phoenix Force would be the salvation of the mutant race, while Wolverine warned Cap about what could happen- the re-emergence of a Dark Phoenix.
Things would indeed go bad, with five X-Men getting the Phoenix Force and going progressively crazier as time went on. Cyclops would go full Dark Phoenix before the Hope Summers and Scarlet Witch were able to wrest the power from him, using it to reignite the mutant race.
6 The Death Of Professor X
One of the biggest consequences of Avengers Vs X-Men was the death of Professor X. The Avengers brought him in, hoping that he would be able to reason with Cyclops... or barring that, use his vast telepathic might to help them in their battle against him.
However, Cyclops was too far gone, too consumed by the power of the Phoenix to listen. Xavier was able to work with Doctor Strange and others to restrain Cyclops, but he lashed out, killing his mentor. Xavier would stay dead for a relatively long time, not returning until 2018's Astonishing X-Men brought him back.
5 Cyclops The Terrorist
After killing his mentor and almost destroying the world, Cyclops was imprisoned. Magneto, Magik, and Emma Frost would break him out and the four of them would go about finding a new group of young mutants to train.
While Scott deeply regretted his actions whilepossessed by the Phoenix, he saw no point in playing nice anymore. While he wasn't mad dog or particularly villainous during this time, he was much more extreme in his actions. He and his team would stay one step of ahead of S.H.I.E.L.D. and the Avengers while still fighting for mutantkind.
4 M-Pox
In the event comic, Infinity, Black Bolt released Terrigen Mists into the Earth's atmosphere. This action would awaken many new Inhumans all over the globe, but would be highly detrimental for mutantkind. Mutants were just coming back from the Decimation, but the M-Pox would put all that in jeopardy. The disease was deadly to some mutants and sterilize others.
This would put the X-Men and the Inhumans at each other's throats. Cyclops would succumb to the disease and Emma Frost would use a mental projection of him to implicate the Inhumans in his death while striking at them.
3 The Country Of Krakoa
In 2019's House Of X #1, it's revealed that things have completely changed for mutantkind and a big part of that is because of the mutant island of Krakoa. Xavier, with the help of Doug Ramsay, made a deal with the island, using the plants it grew to make drugs for the benefit of humanity and mutants.
Using the money and power these industries gave them allowed Xavier and the X-Men power like they've never had before, forcing the nations of the world to recognize them as a sovereign power. Mutants may be a minority of the population, but now they have power.
2 The Five
Another huge change brought on by their time on Krakoa, the Five is a group of five mutants who when they use their powers together can resurrect any mutant, using genetic material supplied by Mister Sinister and mental back-ups of their minds made Xavier.
Death has always been a big part of the X-Men's mission and while death in comics has always been a revolving door, the advent of the Five take the sting out. Mutants can be resurrected immediately after their death and be back on missions with the X-Men. The X-Men have conquered death.
1 The Many Lives Of Moira X
Moira MacTaggert was always thought to be a human scientist, an evolutionary biologist who was befriended a young Charles Xavier and helped him and mutantkind. However, House Of X #2 would change all of that. It was revealed that Moira was a mutant, with a power unlike any other- when she died, she would be born again, starting her life over.
Moira has lived ten different lives. Sometimes she's a hero. Sometimes she's a villain. It's because of her that so much of what has been going on in with the X-Men lately has come to pass. In her tenth life, she's guided Xavier and Magneto, giving them the ideas for Krakoa and setting up mutants as a world power. Without her, none of what's come to pass would be possible.