While embroiled in a major crossover storyline fighting a Secret War on an alien planet Spider-Man acquired an alien costume. It appeared to respond to his thoughts, was able to mimic street clothes and seemed to provide an inexhaustible supply of webbing. Unfortunately, once on Earth, Spider-Man learnt that the costume was in fact a sentient parasite. With the aid of Mister Fantastic, Spider-Man managed to have the costume removed. The costume made one more attempt to bond itself to Spiderman but was forcibly rejected. Weakened by the rejection the costume slithered away, presumed by Spiderman to perish.
Meanwhile, reporter Eddie Brock had been penning a number of articles on the recent Sin-Eater case. Following a false lead, he proceeded to write a series of columns identifying the wrong man as the perpetrator of the crimes. His mistake was revealed when Spider-Man caught the real criminal. Blaming Spider-Man for the downturn in his career subsequent Eddie was found by the alien costume which bonded itself to the failed reporter. The process left both, already emotionally unstable individuals, permanently damaged. Adopting the name Venom the new symbiotic pair decided to take their revenge on Spiderman.
Gifted with strength in excess of Spider-Man's, agility, speed, web-shooters and the ability to remain undetected by Spider-Man's spider-sense the villain has come to be one of the more dangerous enemies the wall crawler has faced. Despite his lethal ability and homicidal hatred of Spider-Man, Venom remains curiously protective of those he considers innocent. He will generally avoid killing bystanders or innocents in his quest for vengeance if it can be avoided. (Of course the fact that he is insane means his definition of innocent as well as necessary casualties fluctuates as the story demands.)