Warning: contains spoilers for Captain Marvel #25!
The X-Men and the Avengers haven't always been on good terms. The events of Avengers vs. X-Men (AvX), a massive crossover event in 2012, memorably centered around the two teams battling for control of the Phoenix Force and the eventual fate of the mutant race. But in Captain Marvel #25 written by Kelly Thompson and with art by Lee Garbett, Belen Ortega and Antonio Fabela, a powerful legacy mutant character takes another step on her journey from villain, to antihero, to X-Man, and now to Avenger.
Captain Carol Danvers has been transported against her will to the dystopian future of 2052. Since then, she's met the children of familiar heroes of the present, learned that most of those familiar heroes have been killed through war and subsequent sickness, and that Namor and Enchantress' son Ove rules over New Atlantis and plans to use Carol's abilities for his own sinister machinations. The future is bleak but not entirely lost, because Carol has allies even in this timeline. Brigid, daughter of Thor, is in her corner, as is an unexpected mutant: Emma Frost.
In Captain Marvel#25, Ove reveals his plan: by bringing Carol into the future, he can travel to the present and begin his rule from there. Carol's appeals to the Enchantress go unheeded and hope seems faint, but suddenly Brigid and Emma Frost arrive on the scene. Emma uses her powerful telepathic abilities to reach out to Luke Cage (choking Carol while under Ove's control) momentarily freeing the Hero for Hire. For her part, Brigid charges into battle and attacks the Enchantress, lunging for the Asgardian while crying out "Avengers Assemble! Even we be only two!"
Though a descendant of the original Avengers would certainly make the designation official, the idea of Emma Frost becoming an Avenger herself would be inconceivable just a few short years ago. In the AvX event, she was squarely on the side of the X-Men, taking orders from Scott Summers in battle. In one memorable scene, she happily punched her on-again off-again lover Iron Man in the gut when her telepathy was inhibited ("Organic diamond meets multi-billion dollar armor," Iron Man thinks to himself as he's doubled over. "The most expensive punch in history"). Even before that, as the White Queen of the Hellfire Club, she was best known as a villain and the X-Men's enemy, wielding her fortune and influence just as expertly as her mutant abilities.
The Avengers are known throughout the Marvel Universe as the de facto team of heroes like Captain Marvel. Though their ranks are many, spots for former villains are quite rare. The powerful telepath who rivals even Xavier joining the team is one moment among many for Emma Frost, who has undergone years, even decades, of character development.