WARNING: The following article contains spoilers for Symbiote Spider-Man: King in Black #1 by Peter David, Greg Land, Jay Leisten, Frank D'Armata, and VC's Joe Sabino, on sale now.
Over the past few years in the Venom series, a lot about the god of the symbiotes, Knull has been revealed. Knull is an ancient being as old as the universe itself and roamed the cosmos for a long time, as he created the symbiotes to be his obedient army. However, over time, the symbiotes turned on him and trapped him in a cage of their own making. Now, Knull has been freed, and he's coming to invade the Earth.
Knull is easily one of the biggest threats in the entire universe, one whose existence rewrote a lot of Marvel history. Symbiote Spider-Man: King in Black takes a trip back to a time shortly after the events of 1985's Secret Wars, where it's revealed that a classic Avengers villain has been trying to stop Knull for a very long time in Kang the Conqueror.
Kang the Conqueror, aka Nathaniel Richards, is a time-traveler from the far future who has clashed with the Avengers on multiple occasions. But while he is generally depicted as a villain, Kang appears to be on the side of good in Symbiote Spider-Man: King in Black #1 or rather, as close to good as Kang can get.
The villain abducts Uatu the Watcher on the moon, and he takes him to his spaceship. There, he straps the cosmic being inside a device and, thanks to his powerful technology, he intends to steal all of Uatu's knowledge. After all, he is a Watcher, an ancient being who has observed humanity for thousands of years. Kang believes Uatu has information that can help him in his quest to save the universe from Knull. When Kang connects Uatu to his computer, the Watcher is able to gain access to all of the knowledge Kang already has: he learns about an ancient battle between the forces of dark and light he never knew about, and he sees a frightening future ruled by Knull that Kang has already experienced one ruled by Knull.
Therefore, in his travels across time, it appears Kang visited a future where Knull has taken over the universe. And although he is generally a villain, even he understands that this is a menace that has to be prevented. Therefore, Kang time-traveled back to the era following "Secret Wars" in order to stop Knull. This means that he has been at this for a very long time, likely longer than anyone else. Now, his plan to steal Uatu's knowledge ultimately fails, but Symbiote Spider-Man: King in Black #1 ends with Kang getting rescued by Rocket Raccoon, long before he would become a member of the Guardians of the Galaxy. The two make for an unlikely pair, but perhaps Rocket will be able to further help Kang in his quest.
Next, Knull will soon arrive on Earth in "King in Black", an event that will see him clash with the Avengers. But while the odds aren't exactly in the superheroes' favor, they might find an unexpected ally in Kang the Conqueror, should the character travel to the present next.