This article contains spoilers for X-Factor #4.
The X-Men have just lost their greatest power in the "X of Swords" event. When Charles Xavier founded the mutant nation of Krakoa, he ensured the X-Men had an ace up their sleeves; they had learned how to resurrect the dead. Consequently, the millions of mutants who have already been slain are being raised to life again. The mutant race is expanding at a race of knots, as fast as the Five - mutants who powers synergize with Cerebro technology to make all this possible - can manage.
This is a dramatic change in the status quo. Death has always been something of a revolving door for the X-Men, but the so-called "Resurrection Protocols" mean that's been formalized. Mutants are a lot less wary about their lives now they know how easy it is to come back; Xavier orchestrated his own assassination to manipulate the mutants, Quentin Quire seems to die almost as many times as he appears, and now even Kate Pryde has been raised from the dead. With the power of resurrection, the mutant community is essentially unbeatable.
The "X of Swords" event has just neutralized that advantage. The action is taking place in a mystical realm called Otherworld, and this week's X-Factor #4 - written by Leah Williams and with art by Carlos Gomez - reveals death is permanent if you die in Otherworld. This is the nexus of all realities, and if you die there, your spirit touches those different realities. Consequently, this introduces a corruption into the Resurrection Protocols, meaning the being created is an unstable composite. Even the Cerebro records are rewritten. Tragically, this lesson has been learned the hard way, with the death of fan-favorite mutant Rockslide.
The X-Men have just lost their biggest advantage. Worse still, nine of them - Magik, Kid Cable, Warlock, Storm, Wolverine, Betsy and Brian Braddock, Gorgon, and Apocalypse - are sworn to engage in a duel to the death with the Swordbearers of Arakko in Otherworld. That means any one of them could die for real - or, potentially, be brought back changed. Because this is the most interesting aspect of the failed Resurrection Protocols; it is possible the Five could unwitting resurrect someone from another reality. While Xavier would be unwilling to experiment with this, it's clear Saturnyne has a far greater plan in motion than just a battle of Swordbearers.
For now, though, the X-Men are left reeling as they realize they are mortal once more. All nine of the chosen Swordbearers are seasoned warriors, and they won't flinch from potential death, but Rockslide's tragic demise will leave the entire mutant community shaken to the core. Emma Frost in particular is devastated; she only joined Krakoa for the sake of the children, and now one of her students is dead. As far as she is concerned, the X-Men's dream will never be the same again.