Venom's symbiote is unpredictable and has on multiple occasions bonded with heroes and villains to create terrifying new versions of popular characters. In one 'What If?' story, the alien symbiote jumped from one Avenger to another Avenger before taking over Thor and wielding his trusted enchanted hammer, Mjolnir.
While Thor's hammer Mjolnir is enchanted so that only those worthy can lift it, over the years other characters have successfully lifted the Marvel superweapon. Among those who lifted Mjolnir include everyone from Deadpool, to Rogue, to Captain America, to even Wonder Woman and Superman. However, Venom's symbiote once got in on the action and took over Thor's body and used the hammer to fight off the Avengers with a powerful blast.
In What If? I#4 by Danny Fingeroth, Mark Bagley, Keith Williams, Tom Vincent, and Ken Lopez, the story takes place in an alternate universe where Spider-Man kept his black suit instead of it bonding with Venom soon after he arrived back on Earth. However, after trying to split with the suit (that's slowly taking control of Spider-Man), it eventually jumps onto the Hulk - while aging Peter Parker into an 80-year-old man. After Peter dies, the Avengers locate Hulk on Mount Rushmore, and the symbiote than jumps from the Jade Giant into Thor.
Thor Venom actually looks pretty cool, as he keeps his red cape and all of his features are turned black or white - including his long hair.
The Venom-ized Thor actually proves to be quite a threat to the Avengers. After Captain Marvel charges at him, he transfers energy to Mjolnir and sends her flying through the air. As the symbiote is about to bond permanently with Thor, the Avengers call in Black Bolt who uses his powerful seismic voice to split the symbiote from Thor, destroying Mount Rushmore in the process. In order to get rid of the symbiote for good, Doctor Strange opens up a dimensional rift to send it away - but before he's successful, Black Cat (still grieving over Peter's death) shoots it with an omni-blaster.
Like many issues from the What If? series, "What If The Alien Had Possessed Spider-Man" moves at a mile per minute. Venom's symbiote spends about three pages bonded with Thor and a lot happens. Thor remains worthy despite the symbiote overtaking his body and he's still capable of shooting off an impressive blast of power - showing he and the symbiote simultaneously worthy. It's sort of terrifying to think what would happen if the symbiote permanently bonded with Thor. The God of Thunder also having the immense strength of a symbiote would be a power that doesn't have an equal in the Marvel Universe. Thankfully, Black Bolt (and then Black Cat) saved the day and saved Thor from being part-symbiote for good. Venom Thor looked cool, at least.
