The legendary artist Alex Ross is perhaps best-known for his work on DC's Kingdom Come, but he's painted his fare share of Marvel Comics superheroes as well - and shepherded a story that would eventually lead to the Marvel Universe's darkest reality. Earth-9997, the world of Marvel's Earth X, is bleak but astonishing, with the world having all but collapsed, villains making power plays - and everyone on Earth has been turned into mutants (or so it would seem). The entire series ran 12 issues and is a fascinating look at a flawed but ambitions attempt to replicate the success of Kingdom Come in the Marvel Universe.
The reasons why anyone would want to create a Kingdom Come-esque story within the Marvel Universe are quite understandable. Kingdom Come centers around Superman and the other Silver-Age heroes leaving after they find the public strongly supports a new group of heroes who aren't afraid to use deadly force in fights. When that same group endangers the United States, Superman returns and a civil war of sorts breaks out among superheroes. This was the story that made Alex Ross a household name, and the time would eventually come when he wanted to tell a similarly-epic story at Marvel.
In the world of Earth X, many of Marvel's heroes are gone; the X-Men, Avengers and all telepaths have been killed upon the birth of a new villain called the Skull, Spider-Man and Wolverine have grown slovenly, and even Captain America has grown despondent (and bald). Norman Osborn is now the President of the United States, and Iron Man lives in seclusion - but has created replicas of his old teammates as the Iron Avengers. In addition, all humans are now mutants and possess powers, and thus the remaining superheroes must navigate a super-powered planet. The entire setting leans heavily into dystopia, even more so than Kingdom Come.
The reason behind all humans obtaining superpowers is eventually revealed to be a combination of the Terrigen Mists mutating Earth's population and further manipulation from the Celestials. The Celestials grow their young inside planets, and manipulate the population of said worlds to become their protectors. A Celestial threat arrives to Earth, but is stopped by Tony Stark, Loki, and the mighty Galactus himself (who in this universe is actually Franklin Richards). Captain America battles the Skull and defeats him by snapping his neck.
Overall, Earth X received a mixed reception from the comic book community. Some found the setting perhaps too bleak and depressing, and much of the book is exposition, courtesy of conversations between Uatu the Watcher and Aaron Stack, the Machine Man. However, Alex Ross' Earth X is a fantastic journey into a possible dark world of Marvel Comics, and though it wasn't as well-received as Kingdom Come, it is nonetheless remembered.