In the upcoming crossover X of Swords, some of the most legendary blades in the X-Men's mythos will return as the mutants of Krakoa unite to face a threat unlike anything they've faced before. However, one of the X-Men's most famous blades isn't actually a physical weapon. No matter which mutant hero is wielding it, Psylocke's psi-blade is a lethal weapon, and it has a more complex history than most of Marvel's physical weapons.
Since the sword is manifested through Psylocke's powers, the key to understanding it comes in understanding the history of Betsy Braddock and the evolution of her mutant abilities. When she first appeared in Chris Claremont and Herb Trimpe's Captain Britain #8, she was just a supporting character in her brother's adventures, and Brian Braddock taking the spotlight as the titular hero while Betsy alternated through various roles.
While Betsy didn't initially seem to have any superpowers, it was eventually revealed that she had mild telepathic and precognitive abilities. These made her a valued member of S.T.R.I.K.E.'s psi-division, but she rarely took center stage in much of the action. Even when she made the jump overseas to join the main X-Men team, Betsy primarily contributed to fights through mind blasts from afar, even fleeing Sabretooth on one occasion when he tried to get ahold of her in a melee.
However, Betsy's powers went through a radical transformation when she went through a body swap with Kwannon, a mutant ninja assassin. While Kwannon shared similar telepathic abilities, but the mind-meld between the two left Betsy with Kwannon's ninja training, finely-honed physical abilities and the ability to produce a psychic knife that was the focused totality of her psychic powers. The weapon would fry a person's nervous system while leaving them physically unharmed and quickly proved its usefulness in battle.
Over time, Psylocke honed her technique, and her psychic blade evolved right along with her. Despite its psionic components, the sword remained accessible to Psylocke even during times that her telepathy was blocked. She once imprisoned the Shadow King in her mind and blocked off her telepathy to keep him in check, yet her psychic knife retained its ability to make psychic contact with those it touched and affect their minds.
The same held true even when Betsy lost her psychic powers altogether. When her telepathic abilities were replaced with telekinetic ones Psylocke's psi-knife only seemed to grow stronger, forming into the longer psi-katana that proved to be an even deadlier weapon. Combined with her telekinetic abilities, the psi-sword could even cut materials on a physical level as well as a mental one. She could shear metal as easily as minds with the sword, and the growth into a new form only proved the first of the weapon's increasing abilities.
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Since Betsy and Kwannon regained control of their original bodies, Betsy has proved increasingly capable of shaping the psi-sword into various other shapes, usually the sword and shield she uses as Captain Britain. Meanwhile, Kwannon can also still produce Psylocke's trademark psychic blade.
Whereas most of the swords teased in the promotional image for X of Swords have been wielded by a variety of heroes over the years, Betsy and Kwannon are truly the only two capable of wielding psychic swords. And now that both mutants are alive and back in their original bodies ,they may just have to duke it out to see which will get to claim the "true" psi-blade in defense of Krakoa.