Randy Schueller, a Spider-Man fan from Illinois actually came up with the idea for the character, Venom. It came from his original costume idea for Spider-Man. Marvel Comics purchased the idea for $220 after Jim Shooter, the editor-in-chief at the time, sent Schueller a letter of acknowledgement for his idea and Marvel’s desire to buy it from him. Mike Zeck then modified Schueller’s idea into becoming the Symbiote costume.
David Michelinie would later write the backstory of Eddie Brock as the alien's new host that became the popular villain Venom. The creature that would become Venom was born to a race of extraterrestrial symbiotes, which lived by possessing the bodies of other life-forms. The parasites would endow their victims with enhanced physical abilities, at the cost of fatally draining them of adrenaline.
According to the Planet of the Symbiotes storyline, the Venom Symbiote was deemed insane by its own race after it was discovered that it desired to commit to its host rather than use it up. The Symbiote was then imprisoned on Battleworld to ensure it didn't pollute the species' gene pool.