Carnage has been going around collecting symbiote codeces. They're the trace that every symbiote leaves whenever they leave a host and that's called a Codex. When he collects all of the codexes he will be able to bring back the villainous creator of the symbiotes: The Dark God Knull. He's framed Eddie Brock and now him and his son Dylan (Who believes Eddie is his brother) are on the run and they're being followed.
The only problem is that the subway they choose to hide in is occupied. Cleedus Casedy just happens to be waiting for them. He still has beef with Eddie Brock for the countless times Venom has beaten down Carnage in the past.
Luckily, there's a person who would gladly rescue Venom from his tough spot between a train and a hard place. That person is the Venom symbiote. Eddie isn't overjoyed to see it as he rejected it after learning it was keeping the secret of Dylan from him ever since they first bonded.
Carnage isn't enjoying this either. He pounces on Venom with his new look. This is a look that relates heavily to Knull himself.
Eddie, Venom and Dylan manage to escape after frying Carnage on the railway line.
They end up going to the one place that the symbiote trusts at this point. The home of Peter Parker aka Spider-Man. They talk where Spider-Man is in utter disbelief that Venom went from a Lethal Protector to god defeating super Alien. They end up going to the Ultimate universe version of Reed Richards known as the maker who Eddie got to remove the venom symbiote.
Spider-Man brings his godson Normie Osborn the son of Harry Osborn. Norman Osborn once bonded with the Carnage symbiote which lead to Norman bonding a portion of the symbiote to Normie, the death of Agent Anti Venom aka Flash Thompson and the capture of Norman Osborn who now believes he's Cleedus Casedy. The Maker has created a machine that can extract codeces. They don't have anyone to test it on though. But there is just one person... Norman Osborn. But when the get there they discover that they're not the only ones looking for him.
But Cleedus Casedy isn't here to kill Norman. There's still so much fun to be had from Carnage's perspective. Carnage just wanted to make friends and Norman Osborn is the perfect candidate for a friendship built from insanity. And so Norman Osborn wears the symbiote once again.