This article contains spoilers for King in Black #4 by Donny Cates & Ryan Stegman.
King in Black proves the X-Men's Jean Grey is Earth's true champion. Knull has come to Earth - the ancient god of the symbiotes, implacably opposed to light and life, and easily the greatest threat the planet's heroes have ever faced. He ruled the cosmos in the days when the universe was young, striving to prevent the evolution of life, even forging a blade that can kill Celestials.
Earth's Mightiest Heroes fell before him. The Sentry was torn in half, Doctor Strange was captured an contained, and even the mutant nation of Krakoa was conquered. A handful of heroes have survived to carry on fighting, going to incredible lengths - but even that didn't seem to be enough. And then, in King in Black #4, a single mutant superhero stepped out of the shadows - and did more damage to Knull's plans than anyone else.
Jean Grey is an Omega level telepath, a mutant whose dominant power is deemed to register or reach an undefinable upper limit of that power's specific classification. In King in Black #4, she uses her telepathic abilities to coordinate an offensive against Knull, helping Venom's son Dylan disrupt the symbiote hive - and freeing the heroes who had been imprisoned within the hive. She instantly downloads what Doctor Strange describes as "a rather... intense plan" to drive Knull and his forces back to the center of New York - and unleash all the armies of Atlantis upon him, swiftly followed by bombardments of lightning courtesy of Storm and Thor. For the first time, Earth's heroes are actually on the offensive, and it's all thanks to Jean Grey. Jean is even able to press the advantage, raiding Knull's mind and discovering his one weakness - the Enigma Force, the yin to Knull's yang, the light to his darkness.
The modern X-Men comics have actually reduced Jean Grey's power, retconning it so that she is an Omega telepath but that her telekinetic powers do not operate on the same scale - despite the fact she has previously been able to knock down Galactus even without the Phoenix Force. Still, though, Jean is clearly still Earth's greatest champion; she may not possess the brute strength of Thor, but her telepathy allows her to coordinate others in something akin to the "Battle Meld" seen in the old Star Wars Expanded Universe. Thor may have been able to break Knull's jaw, but Jean is the one who leaves him shaken, who even penetrates the god of the symbiotes' psychic defenses and discovers his secrets.
There is a sense in which the X-Men left Earth vulnerable; they provided only a handful of heroes, and then retreated to cut their losses, only to find Krakoa itself conquered when the King in Black bonded with a mutant. But Jean Grey is not one to stand idly by, and her dialogue suggests she's had to get to New York the long way round, arriving in time to telepathically evaluate the state of play and quickly put together a remarkably effective plan. There is rich irony in the fact that, although mutants are typically hated and feared, Earth's foremost champion in King in Black is one of the X-Men.