The alien symbiote was captured and leveraged during research to try and cure Harry Osborn of his Oshtoran Syndrome, but it does much more than that. The symbiote grants Harry various powers, making him nearly as capable as his friend Peter in fights. When Kraven stabs Peter, the symbiote bonds with him to save his life. Peter enjoys using the newfound powers granted to him by the symbiote, but it becomes obvious that it’s corrupting his emotions, leading him to a brutal fight with Miles. Peter finally removes the symbiote, but it bonds with Harry once again and becomes more aggressive. Together, they become Venom, a violent alien creature who attacks the city, killing Kraven, and spreading the symbiote influence throughout New York.
Thousands of New Yorkers are forcibly bonded with symbiotes, but the Spider-Men are able to stop Venom, thanks in large part to Peter’s past bonding with the symbiote and Mister Negative’s assistance to become Anti-Venom. Miles and Peter occupy Venom while Mary Jane destroys the meteorite that amplifies their influence. Following the fight and destruction of the meteorite, they’re able to remove the symbiote from Harry, but Harry is badly injured, leading Norman Osborn to develop a crusade against the Spider-Men.
Source: www.gameinformer.com