When Spider-Man was on the Beyonder's "Battleworld", he acquired a black costume which turned out to be an alien symbiote. Spider-Man rejected the symbiote, which then latched onto ex-Daily Globe columnist Eddie Brock. Eddie thus became "Venom" with powers akin to Spider-Man along with limited shapeshifting and camouflage capabilities.
After Brock, numerous other hosts for Venom followed; the most notable being the villain Mac Gargan (formerly known as the Scorpion) who fought as part of the Thunderbolts during the Civil War, and Flash Thompson, who became the military-styled superhero 'Agent Venom'.
When Venom (with Mac Gargan as its host) attacked Eddie Brock, the symbiote tried to return to its earlier host. This triggered Brock to become a new white symbiote 'Anti-Venom'. The new symbiote was created when remnants of the Venom symbiote in Brock's body merged with special antibodies in his blood created from Martin Li (Mister Negative)'s Lightforce energy. Having turned into Anti-Venom, Eddie Brock almost killed Mac Gargan.
Mac Gargan recovered and fought against the Skrulls in the Secret Invasion. Afterwards, Norman Osborn (the Green Goblin) transferred Mac Gargan from the Thunderbolts to his new Avengers team, the Dark Avengers. Having used a medicine that made the symbiote less montstrous, Osborn had Gargan pose as Spider-Man on the new team.Venom (primarily the Eddie Brock incarnation) has also been featured in various media adaptations of Spider-Man over the years. The character was portrayed by Topher Grace in Spider-Man 3 (2007), and by Tom Hardy in both Venom (2018) and Venom: Let There Be Carnage (2021).