The spawn of the alien symbiote known as 'Venom,' Carnage was forged from having had retained the genetic information of Spider-Man and the criminal Eddie Brock. However, its extra power came from its choice of host - a psychotic serial killer named Cletus Kassidy. Having had emerged from a troubled childhood, Kassidy was once an orphan who tortured the family dog and killed his grandmother by pushing her down a flight of stairs. Sent to a home for boys, he burned the institution down after killing both the disciplinary administrator and a girl who refused to date him. Diagnosed as a homicidal maniac, Kassidy was given 11 consecutive terms at Ryker's prison. Eventually, he ended up in a cell with Eddie Brock, the host of Venom, a supervillain who was defeated by Spider-Man. Sometime later, Brock's symbiote launched a jailbreak to free its human host.
Pregnant, the asexual alien entity gave birth to a red offspring during the rescue. Acquiring a host, the newborn symbiote bonded with Kassidy through a cut, allowing it to enter his bloodstream. This instantly granted the killer incalculable super-powers, and the strength to escape the penitentiary. With his new-found freedom and superpowers, Kassidy began a Manhattan-wide killing spree. These murders included choosing targets at random, such as selecting victims from the telephone book. He would mark each vicious act by writing "Carnage" on the walls in his own blood, which became the name of his new identity.
Confronted by Spider-Man, Carnage easily defeated the web-slinging hero due to possessing far superior speed, agility, and reflexes. He even showcased the ability to web-crawl, regenerate, and shape-shift. This not only enabled Kassidy to morph the symbiote into clothing, but change his hands into bladed weaponry and form tentacle-like appendages to restrain his victims. Frightening and seemingly invincible, Spider-Man formed a desperate truce with Venom, who he had reluctantly agreed to free from his island prison.
Together, they took on the serial killer - finally defeating him through the use of a sonic based attack formed by the stage equipment of a nearby rock concert. Despite the symbiote having appeared dead, and Kassidy imprisoned at the Ravenloft Asylum, the murderer soon escaped. This was made possible by the alien lifeform having survived in his bloodstream.
Carnage was one of the inmates that had broken out of Ryker's during the super villain breakout.
- In one of the trailers of LEGO Marvel Super Heroes is a scene of Venom breaking out from jail, but in the game he was replaced by Carnage.
- He has the same character movement and abilities (except Big Figure transform) as Venom.