Eddie Brock shared a prison cell with serial killer, Cletus Kasady for a short period of time. As Eddie broke out of the cell, a piece of the alien symbiote was left behind. It bonded with Kasady and created Carnage. During the 14 part Maximum Carnage storyline, many Marvel superheroes were involved in stopping Carnage and his team of super villians from destroying NYC. Below are sites that go into greater depth describing Carnage's history: