The Post-Krakoa Era of X-Men is in full swing; time for a crossover that fragments and divides Mutantkind because god knows the ‘Lost Decade’ didn’t do that for ten years before House of X.

(From Marvel’s PRESS RELEASE)

New 'Raid on Graymalkin' Crossover Pits X-Man Against X-Man for the Future of Professor X's Dream

'Raid on Graymalkin,' a 4-part crossover between Jed MacKay and Ryan Stegman’s 'X-Men' and Gail Simone and David Marquez’s 'Uncanny X-Men,' begins this December.

The X-Men’s new era is fully underway, and starting this December, fans will experience an epic crossover between two of its three flagship titles in "Raid on Graymalkin!"
Left broken and scattered in the wake of Krakoa, the X-Men have since regrouped into distinct teams of their own. In Jed MacKay and Ryan Stegman’s X-MEN, Cyclops leads a squad of hardened mutant soldiers out of an abandoned Sentinel factory in Alaska. While in Gail Simone and David Marquez’s UNCANNY X-MEN, Rogue attempts to build a new home with her chosen X-family along with a handful of new students down in Louisiana. Their different approaches to the X-Men’s mission erupt into a tense standoff when a plan of action is needed to take down Graymalkin Prison, formerly Xavier’s Institute.
The X-Men’s school has been transformed into a brutal penitentiary for mutants by the callous Dr. Corina Ellis, and her prize inmate is none other than Professor X himself! But after his actions during Krakoa’s fall, is Professor X worth saving? And who or what is the secret weapon that Ellis is confident will protect Graymalkin against any attack? Find out in this four-part mutant milestone that solidifies the X-Men’s post-Krakoan world!

"It's been an exciting time being part of the launch of the line of X-titles, but now we're really getting into it- our first crossover!” MacKay shared. “Raid on Graymalkin is going to push the X-Men of both Alaska and Louisiana to their breaking points and set the tone for the future, and it's been great to work with a legend like Gail in bringing it to life!"
"Writing in the X-World has been one of the most fun and rewarding experiences of my career," Simone said. “Best characters, best art team, best editors and best fellow creators, particularly Jed Mackay, who is just an idea machine. We've been throwing ideas back and forth nearly every day since we all started putting together the bones of what the X-Verse would be like, and I think it helps tremendously that we like and respect each other's work.”
“I hope that this is a precursor to more short crossovers,” Simone added. “We love the idea of doing something impactful, where it feels punchy and fast-paced, with some genuine, lasting effects. You can still just read X-MEN or UNCANNY X-MEN and follow the story, but you'll get a bigger, richer picture if you get both. As always, we have the welcome mat out for newbies and long-time readers alike!”

Sources:

https://www.marvel.com/articles/comics/x-men-uncanny-x-men-new-raid-on-graymalkin-crossover