Carnage, about to birth another symbiote, was threatened by its existence and attempted to destroy it. Venom, however, wanted to raise and train the new symbiote as an ally and partner. Weakened by the actual birthing, Carnage was unable to kill the newborn symbiote. Supplanting the symbiote in Patrick Mulligan, a New York City cop, it returned in later days. Venom named the new symbiote Toxin, after himself. Carnage and Venom both knew that because of how powerful Toxin will become, and also since his host is a Police Officer, he would most likely be a Hero rather than a villain, because of this they made a temporary alliance so they could kill him, but in the end they were defeated.
Toxin aided Spider-Man and the Avengers in rounding up many of the escaped convicts from the raft including, King Cobra, The Wrecker and The Answer.
After Razorfist (another escapee) killed his dad, Toxin went on the hunt for him. Before battling Razorfist, Pat said to Spider-Man "If I cross that line, throw me in Rikers and throw away the key."
Toxin defeated Razorfist, and, despite Razorfist's taunts, he refused to kill him and finally gained control of the beast inside him. After defeating Razorfist, Pat made a deal with Toxin. Pat would have complete control of Toxin's powers and Toxin could have his "play time", but only for two hours and no criminal activity.
Pat got back together with his estranged wife Gina and told her about Toxin and about the reason he had left her.
Red and Blue Symbiotic Costume: Toxin possesses the "unique" special abilities of his two symbiote predecessors. He can stick to walls, can change his identity to that of a completely different person and also has unlimited webbing, which takes on a shape of a steel chain in his case. Toxin can also blend in with its surroundings and become undetectably camouflaged, and he can form solid weapons from his limbs like his father. Toxin also seems to have some sort of quick-healing ability like his predecessors, as his wounds from his first battle with Razor Fist healed instantly. Unlike the others, Toxin can track anyone, not just other symbiotes, as long as he has something to begin from, within the entire city of New York and possibly further.
Unlike the other symbiotes, Toxin does not try to take over the mind of his human host. Instead, the symbiote actually thinks and voices its opinions to the human host (as evidenced in Toxin's Cut to the Chase storyline). The symbiote usually speaks when the host is in his "human form". In addition, he seems to have a much stronger resistance to sonics and intense heat compared to Carnage, though this does not mean that Toxin is more resistant to violent urges: he can get carried away with violence and fighting crime, even if it's a petty crime.
When he's not angry or fighting, Toxin is slim and smooth-lined, although still well muscled, closely resembling Spider-Man in his Symbiote costume. When he becomes upset or aggressive, he grows into his much bigger form and he gets even stronger, with vicious fangs and long curving claws.
As he has only been born, Toxin's powers while incredibly vast, are still growing, and he will become much stronger and much more formidable as he grows with time.
Superhuman Strength: He possesses vast superhuman strength, and is stronger than Carnage and Venom combined. Depending on how angry or upset he gets, he grows to a much larger size, with large claws and vicious fangs. He was strong enough to defeat both the Wrecker and Piledriver at the same time without effort.
Superhuman Speed: He can move or run at speeds greater than any normal human being.
Superhuman Stamina: He can exert himself for at least 48 hours until fatigue impairs him.
Superhuman Agility: His agility is superior to any normal human being.
Superhuman Reflexes: His reflexes are superior to any normal human being.
Superhuman Durability: He is much more resistant to injury than Spider Man, Venom and Carnage combined but still has the traditional symbiotes weaknesses but to a very minor degree. The tissue the symbiote is composed of renders Mulligan's body much harder and much more resistant to physical injury than the body of an ordinary human. Mulligan can withstand high caliber bullets, great impact forces, powerful energy blasts, and falls from great heights without sustaining physical injury. He is durable enough that he can withstand multiple barrages of high caliber weapons such as machine guns from close range completely unperturbed, can withstand blows from superhuman beings such as Wrecker, Spider-Man, Carnage, etc unharmed. He also possesses an extreme resistance to pain, and seems to feel it only to a certain extent.
Camouflage Capabilities: The symbiote is capable of mimicking the appearance of any form of clothing, camouflaging with its surroundings, and even mimicking other people.
Wall Crawling: Much like Spider-Man, Toxin has the ability to cling to virtually any surface. He can rapidly crawl, walk, or run across even slick/wet surfaces.
Webbing: Toxin can project a powerful, organic web-like substance similar to the webbing Spider-Man has. He typically uses this webbing to swing from building to building or as a means of immobilizing someone. The webbing is incredibly strong.
Regenerative Healing Factor: He can heal at speeds much faster and much more efficiently than a normal human.
Former Weaknesses (Toxin)
Sonic and Thermal attacks but to a much lesser degree compared to the other symbiotes.
He was a police officer and has had formal training.
Toxin is superhumanly strong and can lift within 44 - 80 ton range, this is not his true limit as his strength increases with his size and variable muscle mass. His strength is stated to be greater than Carnage and Venom combined; thus enabling him to lift over 44 tons in a calm state. Toxin's size seems to fluctuate depending on his level of anger.
Despite the fact that Spider-Man has teamed-up with Venom on numerous occasions, Toxin appears to be the first symbiote that Spider-Man considers an ally.
Toxin has two facial appearances: He can form a jaw with sharp teeth, or he can go without one. When he goes without a mouth, his appearance looks like a cross between Spider-Man and Deadpool. With a jaw, he resembles a Carnage-like mouth and Venom-like teeth.
Rick Remender, the writer of Venom Vol 2 confirmed that Patrick had been killed by Blackheart who "beat him to death in a gutter".
Personality and character-wise, Toxin is very similar to Hybrid, his predecessor.
Compared with the other symbiotes (She-Venom, Scream etc.), Toxin is considered to not be a "carbon copy" of his predecessors.
The symbiote inside him, having been just spawned, acts very immature and childlike, once refusing to aid Pat in battle until he apologized for an earlier argument, and even turned him into a scrawny weakling he called "mild mannered Larry" during his battle with Razor Fist, and broke his laptop after he saw Pat call him a "monster" on a blog and later stole a bag of laptops to replace it, only to be stopped by Spider-Man.
Toxin is a second "hero" in the symbiote family after Hybrid.
The reason Toxin behaves differently than other symbiotes is because every 1,000 generations, a symbiote is born with genetic memory breakdown, which is basically symbiote amnesia, and must be taught by an older symbiote to correct the problem or be destroyed. Since he was abandoned right after he was born, he never learned how to act like a 'true' symbiote.