WARNING: The following contains spoilers for Empyre: X-Men #2 by Gerry Dugan, Ben Percy, Leah Williams, Lucas Werneck, Nolan Woodard, VC's Clayton Cowles, and Tom Muller, on sale now.
The current status quo in the Marvel Universe has Charles Xavier reaching out to any and all mutants who wish to join the nation of Krakoa. This has seen former enemies join the psychic's side, including Magneto, Mr. Sinister, and even Apocalypse.
In Empyre: X-Men #2, it's shown that the Shadow King, another of Xavier's most fearsome psychic foes has joined the X-Men's ranks, too. Known for his corpulent form, grievous grin and mental abilities on par with Xavier's own, the Shadow King is one of the oldest and most powerful mutants on the planet. He's also usually also one of the most feared, making his place as an ally of his old foe extremely surprising.
The issue deals with the fallout of Scarlet Witch trying to undo her greatest mistake: the decimation of the mutant race. Unfortunately, those killed by her had been brought back on Genosha as undead zombies. Making matters worse is the presence of both the invading alien plants known as the Cotati, as well as the botanist terrorists known as Hordeculture. This leads to a massive battle, one in which Hordeculture uses their pheromones to make Angel fall in love with and protect them.
The presence of the Cotati is especially troublesome given Krakoa's status as essentially a gigantic plant organism. This sees Black Tom using his own mutant abilities to get the planet to defend itself, but the invaders are still an overwhelming force. In desperate need of reinforcements, Magik calls out for any and all psychics to come to the rescue. Black Tom opens a portal for the cavalry to come in through, and there are a few notable villains along with expected psychic X-Men like Quentin Quire, Emma Frost, and the Stepford Cuckoos, there are also a few notable villains. These include Mastermind, Mr. Sinister, and the spider-legged, ever grinning visage of the Shadow King.
The Shadow King was first introduced in X-Men #117 and was created by Chris Claremont and John Byrne. He is an ancient astral entity of great psychic power, and in the past has been a threat to minds as great as Charles Xavier's. He eventually became bonded to the mutant Amahl Farouk, who became a petulant and corpulent crime lord in Egypt. There, he met Xavier, who was disgusted by the man's petty use of his mutant abilities. Xavier's subsequent defeat of the Shadow King incited a great fear and hatred towards Charles.
The character last appeared in Charles Soules' run on Astonishing X-Men, warring again with Charles Xavier. There, Xavier had recently come back to the world of the living, now simply going by the name X. X was using the body of Fantomex, who had sacrificed his mind to allow Xavier to leave the astral plane. Unfortunately, the Shadow King had escaped as well, using X as a vessel before literally ripping himself out of his head.
Shadow King quickly decimated the X-Men, but X was able to revive himself soon enough to reach out to Psylocke. Together, they defeated the Shadow King, with Professor Xavier back for good and in control of his body. From there, he would go on to lay out his plan for mutantkind on the nation of Krakoa.
Since then, Farouk's body, or at least a version of it, was likely cloned, with his psychic essence implanted into it from there. His smile is as dastardly as ever, making it unclear how much he's changed for Xavier to trust him. As mentioned, other villains such as Mastermind and even Mr. Sinister are also there, so somehow, Xavier is seemingly able to keep even his most fearsome and outright evil opponent on a tight leash for the greater good. It's possible that, seeing how much power and influence Xavier has amassed, as well as his overall abandonment of his old ideology, Shadow King now willingly serves this new and arguably improved Professor X. Until his intentions and ambitions are made clear, however, his standing alongside Emma Frost and Quentin Quire is more than a little alarming.