One day after a fight with Spider-Man in New York City, carnage got flung into outer space and was perpetually moving for about a year, and then he landed on the back of Galactus
Galactus had an eaten in a while, so his power was starting the weaken carnage, took advantage of that and absorbed his power using that power he traveled back to earth
Unknowingly Carnage opened a portal to another universe, started pulling people from other universes into his and started taking them out
Spider-Man got a group of these people and people from his universe to take out carnage and end this war that he has started
This group of people went in for the attack and it took them quite a while. They lost a few soldiers, but in the end they came out of victorious.
After his defeat carnage power was pretty much drained and Deadpool put a bullet right in his skull, killing Cletus inside of carnage