Marvel is home to plenty of terrifying locations, and the fact that there are an endless number of realities and timelines beyond those of the primary Marvel Universe only means that there are even more opportunities for gut-wrenching horrors to abound. For every threat on Earth there is another out among the cosmos, in other realms such as Muspelheim, or beyond the veil of realities such as the Cancerverse. This version of the Marvel Universe is undoubtedly one of the worst imaginable, and it all came to rise when Captain Mar-Vell overcame mortality itself.
The Cancerverse was first introduced into the greater tapestry of the Marvel Multiverse during 2010's Realm of Kings #1 by Dan Abnett, Andy Lanning, Leonardo Manco and Mahmud Asrar. Following the then-recent battle between Black Bolt and Vulcan out in space, a hole had formed in reality itself that came to be known as the Fault. Using Knowhere as a base of operations, Doctor Gruenwald from Project P.E.G.A.S.U.S. joined Richard Rider and Quasar in studying the anomaly, the latter of the three even venturing into the Fault safely by changing his physical form into one of pure energy. On the other side Quasar finds the remnants of a world much like his own, one that has been overrun by the Eldritch gods now worshiped by Earth's Mightiest Heroes and anyone else that remains alive. As strange as it sounds, the worst part of this reality is that the number of people who have survived to witness this nightmare just happens to be everyone.
The Cancerverse was at one point an almost picture-perfect mirror image of the primary Marvel Universe, at least until this reality's Mar-Vell discovered he was dying of cancer. After seeking out every medical and supernatural treatment and finding them all to be futile, the cosmic hero could only wait for death. Or so it seemed until the Many-Angled Ones came to offer him immortality. These ancient dark gods from the space between realities told Mar-Vell that by destroying Death he could save himself, and with the powers bestowed by them Mar-Vell corrupted his allies among the ranks of the Avengers and Defenders to do just that. Unfortunately, destroying Death itself meant that the Many-Angled Ones were free to invade Mar-Vell's universe and eternally corrupt the unending life that now inhabited it. Upon Quasar's discovery of what had happened came a new kind of threat in that the Cancerverse had become aware of the primary reality, and their own Quasar proved just as capable of traversing the Fault unharmed.
The story continued in the pages of The Thanos Imperative mini-series by Dan Abnett, Andy Lanning and Miguel Sepulveda, with the Guardians of the Galaxy joining forces with the likes of the Shi'ar Empire, Silver Surfer and even Thanos himself to fight back the invading forces of the Cancerverse. Lord Mar-Vell was quick to send his own twisted Avengers, the Revengers, through the Fault and into the primary Marvel Universe where they quickly set their sights on and kidnap Namorita. When the heroes venture into the Cancerverse for themselves, the discovery that death does not exist among this metastasized reality means that their enemies cannot be felled by their own hands. Luckily, Thanos is capable of ending the lives of the Cancerverse's inhabitants thanks to his status as the Avatar of Death in his own reality. Of course, this just means that he is the prime target for Lord Mar-Vell, as killing Thanos would mean another universe free of death to be conquered. That would make sense, but killing Thanos means that Death has to come for him, and when it does it is standing face to face with Lord Mar-Vell, who soon enough learns that cheating Death is something that you only get to do once.
In an instant, Death obliterates not only Lord Mar-Vell, but all of the Cancerverse inhabitants that had escaped it to invade the primary Marvel Universe. This also seals the Fault once and for all, saving all of reality and ensuring that both life and death continue on as normally as they can in this particular reality. The Cancerverse is far from gone or forgotten, even if its heyday is long past. Most recently it has even been weaponized within the pages of Heroes Reborn, proving that no matter what reality itself looks like, the Cancerverse is truly the most horrifying version of the Marvel Universe that has ever existed.