Toxin (Patrick Mulligan) is a fiction character, a superhero in the Marvel Comics universe. He is the third major symbiote of the Spider-Man series, the ninth known to have appeared in the comics outside of the Planet of the Symbiotes storyline, and the first symbiote that Spider-Man considers an ally, despite temporary alliances with Venom on numerous occasions and an alliance with Anti-Venom (the second symbiote he has allied himself with).
Much like his "father" Venom, Carnage produced offspring: a third symbiote. Carnage felt only dislike and hatred towards his new spawn, even before he "gave birth" to it, both fearing that it could become much stronger than himself, and being generally disgusted at the thought of giving birth (due to Carnage being male). At the same time, Carnage's rival and "father" Venom became aware that Carnage was "pregnant" and sought out Carnage to talk about this new symbiote. Carnage had resolved to kill his spawn as soon as it was born. Venom was doing his best to protect the new symbiote, with the intention of raising it to become his new partner. Venom was also concerned that, as the 1,000th symbiote of their line, the new symbiote could potentially become psychotic because of genetic breakdown.
After the birth of his "son", Carnage felt too weak to kill his newborn (due in no small part to his attempt to repress the birth after burying himself). Since he could not kill his newborn, he implanted it into Patrick Mulligan and fled, intending to kill him later and destroy the new symbiote when he was up to strength.
Prior to being implanted with the alien symbiote, Patrick Mulligan was one of New York City's finest cops, yet he was also troubled with various personal problems. His stressful job, along with the strain of his new family (his wife, Gina, was pregnant with their first son, Edward) were taking their toll on him. One night, while on duty, he came across the site where Carnage was giving birth to the new symbiote. Carnage, needing a host to hide the symbiote to keep it from Venom, bonded it to Patrick. Carnage resolved to kill both Pat and the symbiote currently hiding in his body since Toxin wasn't yet strong enough to manifest in a super-powered costume like the Venom and Carnage symbiotes. Venom was still hopeful that he'd be able to use this new symbiote as a partner and fought Carnage to a standstill to protect Pat and the symbiote whom he christened Toxin, after himself.
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Official Site: Marvel
Links: Toxin Wiki Entry Spider Fan Marvel Database