The Cancerverse, for those not in the know, is the universe on the other side of the Fault. The Fault is a giant tear in the fabric of space and time that was created when Blackbolt and Vulcan both died in a giant bomb detonation…..the detonation created it, not their deaths.
Anyway, that universe was very similar to 616 up until the death of Mar Vell, the original (Marvel) Captain Marvel. Dieing of cancer, he died surrounded by those he knew, cared about, and fought beside. That’s how it’s supposed to happen, right? Well, on the other side of the fault he sacrificed everyone around him to summon the avatar of Death and kill it, thus killing Death. From there life grew like a cancer across the universe, the Many Angled Ones now in control. All because Mar Vell had to go pull a Universe X on everyone.