The following contains spoilers from Thor #28, on sale now from Marvel Comics.
Marvel's popular anti-hero Venom has been a hit since his debut in the late 1980s. Born from Spider-Man's rejected living costume that the wall-crawler obtained during the first Secret Wars (and clearly introduced as a villain), many writers have redirected Venom towards heroism in recent years. Although the symbiote is most often associated with its most common human host Eddie Brock, the alien life form is most interesting when possessing the existing superheroes of the Marvel Universe, including the Mighty Thor.
In Thor # 28 (by Al Ewing, Donny Cates, Salvador Larroca, Edgar Delgado, and VC's Joe Sabino) the God of Thunder and Venom battle a newly spawned alien symbiote, Bedlam's Son, which is controlling a mindless Darkoth. In order to effectively battle this childlike force of destruction, Thor allows the Venom symbiote to join with him. However, this is not the first time Thor and the Venom symbiote have merged in a Marvel Comic. In fact, 1989's What If...? # 4 (by Danny Fingeroth and Mark Bagley) featured a surprisingly similar pairing that shares parallels with current events.
How Thor and Venom Merged in What If...
What If...?, always a great platform for writers and artists to play with drastically altered continuity, featured the story "What if the Alien Costume had Possessed Spider-Man." In the story, Peter Parker waits too long to try and separate himself from the living black costume and Reed Richards is unable to break the bond between the two. The suit slowly drains Parker's energy, then goes seeking other powerful victims to expand it's power, possessing the Hulk before turning its attention to Thor. The God of Thunder is initially overwhelmed and succumbs to the costume, but Richards develops a sonic weapon powerful enough to split the two.
During this bonding, the symbiote was exclusively thirsting after power and energy and its takeover of Thor was a means to that end. This strongly implies that the alien's original plan was to absorb the heroes of Earth until it became unstoppable, had it not permanently bonded with Eddie Brock and developed into the Venom persona.
The Latest Thor/Venom Merging Was Different But Still Similar
The most recent 'Thornom' bonding was a mutually agreed team-up, with Thor, Venom, and Brock all maintaining their specific identities and working together in their battle against Bedlam's Son. The Odinson's incredible strength and control of lightning, enhanced by Venom's alien might and power proved to be an unstoppable force against their new antagonist, although the symbiote still proved to have ulterior motives.
Although the recent story eventually reveals that there is more to the temporary bonding than meets the eye, it is still a reflection of the classic issue. In both cases, the alien force is in desperate search of greater power (albeit for different reasons) and finds it in the form of the God of Thunder, creating a new, even more powerful force of nature in the process.