Cletus Kasady was a severely mentally unstable serial killer who briefly shared a cell with Eddie Brock. When Eddie broke out, he left a part of his Symbiote behind, and Carnage was born. Using the abilities of his new symbiote, Cletus created a long history of slaughtering people and being put in jail for it.
Many of his adventures involved Venom and Spider-Man, whom he believed to be his mortal enemies. During one such stint in the Raft, a mass breakout occurred, and Carnage was flown into orbit and ripped in half by the Sentry, seemingly being killed. Years later, in an attempt to recover Kasady's symbiote, his body was captured, and he was found to be alive. Given cybernetic implants for legs, Cletus eventually recovered his symbiote as well and remains at large.
The name "Carnage" may also refer to the Carnage Symbiote, the spawn of the Venom Symbiote, which bonded to Kasady while he was serving eleven life sentences in prison.