X-Men: The Last Stand (2006)

The human government develops a cure for mutations, and Jean Gray becomes a darker uncontrollable persona called the Phoenix who allies with Magneto, causing escalation into an all-out battle for the X-Men.

When a "cure" is created, which apparently can turn any mutant into a "normal" human being, there is outrage amongst the mutant community. While some mutants do like the idea of a "cure", including Rogue, many mutants find that there shouldn't be a "cure". Magneto, who still believes a war is coming, recruits a large team of mutants to take down Warren Worthington II and his "cure". Might seem easy for the X-Men to stop, but Magneto has a big advantage, which Wolverine doesn't have. Jean Grey has returned, and joined with Magneto. The Dark Phoenix has woken within her, which has the ability to destroy anything in her way, even if that "anything" is an X-Man. - rsanchez

When a special medicinal "cure" for mutants is created by a government funded laboratory, a choice is to be made by mutants all over the world. However, this does not sit well with Professor X and his X-Men, who are questioning about the situation. But Magneto sees this news as a threat, and begins to build a mutant army in order to destroy it. On the other hand, in light of the returning Jean Grey, the X-Men find that they are not only up against Magneto and his Brotherhood, but a deadly force more powerful than anything that came before. - Blazer346

After the uneasy alliance with Magneto in X2 (2003), the entire species of the powerful meta-humans find themselves before an agonising dilemma: with the aid of a revolutionary medical treatment, all mutants can now suppress their extraordinary gene, to live an ordinary life as humans. To further complicate matters--as the former X-Men member, Jean Grey, evolves into the dark and omnipotent Phoenix--Magneto amasses an invincible army of mutated supporters, against the backdrop of odious conflict within the mutant community. At first, the super-humans were fighting for acceptance, then, for survival. Now, all mutants must choose sides. Can Professor X, Storm, Wolverine, and a handful of loyal X-Men stop the formidable menace, Phoenix? - X2 (2003)

The X-Men, mutant heroes sworn to defend a world that hates and fears them, are back. This time, with the help of new recruits Beast and Angel, they must face evolution in the form of their former teammate, Jean Grey. Possessed with the cosmic power of the Dark Phoenix, the resurrected Jean Grey has become a danger to herself, her mutant comrades, and the entire planet. To stave off this imminent threat to humanity, a potential "cure" is discovered and processed to treat, and ultimately eliminate, genetic mutations, once and for all. Now, as the battle lines are drawn, the X-Men, led by Professor Charles Xavier, must contend with Jean Grey's world-consuming powers, as well as the malevolent Brotherhood, a band of powerful mutants organized under Xavier's former ally, Magneto. - comicfan

When a private laboratory, supported by the government, finds a "cure" for the mutants, using the D.N.A. of a powerful boy, the mutants have the option of giving up their powers and become human, but their society split. Magneto opposes and decides to join a force to fight against the government, and kill the mutant boy. Meanwhile, Jean Grey resurrects uncontrolled by Xavier and with the personality of the powerful Dark Phoenix. She destroys Cyclops and Professor Charles Xavier, and allies with the evil forces of Magneto, making them almost invincible. - Claudio Carvalho, Rio de Janeiro, Brazil