This article contains spoilers for X-Men #10.
Vulcan, brother of Cyclops and one of the most powerful X-Men, is back - but with a dark and twisted secret. There was a time when the ruthless Mr. Sinister believed Cyclops and Jean Grey to be the key to the future of the entire mutant race, and it's easy to see why. The Summers family tree has become one of the craziest in comics, spanning time and space, including some of the most powerful mutants ever to walk the Earth.
Gabriel Summers, aka Vulcan, is easily the most dangerous of the Summers family. An Omega level mutant, he can manipulate energy on a cosmic scale, and he's never been the most mentally stable of individuals. He absorbed the energy released by the Scarlet Witch's Decimation spell, and became powerful enough to conquer the entire Shi'ar Empire. Ruthless and warlike, Vulcan led the Shi'ar into a war that brought the entire galaxy to the brink of destruction before he was apparently killed, plunged into a dimensional breach called the Fault. But he's returned as part of Jonathan Hickman's X-Men relaunch, mysteriously resurrected and with his psychotic mind apparently stabilized. Even the X-Men don't understand how he's come back, but that mystery is finally explained in this week's X-Men #10.
The Fault tore a hole through the Multiverse, and it seems that tear was close enough to another dimensional plane. Monstrous beings plucked Vulcan out of the Fault, and they cast a covetous eye upon his power, seeking to turn him into a tool for their own purposes. These beings were never identified, but they possessed enough power to tear Vulcan apart on a molecular level, and to peer into his soul and assess his potential for good and for evil. Disturbingly, they are only interested in the latter, considering the potential for good to be a flaw in his existence. They rewrote Vulcan's mind, creating the "good" exterior he has been presenting to the X-Men all this time, a false identity beneath which was subsumed his true persona. And in X-Men #10, the true Vulcan finally rises to the surface - with explosive results.
Of course, this revelation raises a lot more questions than it does answers. It seems Vulcan has been transformed into a weapon for creatures that care nothing for good, and whose purpose on our dimensional plane is surely only malevolent. The power levels they display appear to correspond to the Elder Gods, explaining why they are able to effortlessly hold Vulcan prisoner. These quasi-demonic creatures were banished from Earth millennia ago, and there's already been speculation they could return in the upcoming "X of Swords" event, with a Chthonic mask seen worn by one of the Swordbearers of Arakko.
Whether Vulcan is somehow tied to "X of Swords" or not, the fact remains that one of the most powerful mutants on Earth is actually a twisted, broken being whose true nature has been concealed from all the telepaths of Krakoa. Vulcan conquered an entire alien empire once before, and he held his own against some of Marvel's cosmic champions, meaning he has become the greatest threat to the mutant nation - in part because the X-Men have no idea he's a problem.