Knull was first mentioned in a Marvel comic book in 2013, and didn’t make his actual on-page debut until 2018, but he’s really been around since the very beginning of the Marvel Universe. In fact, he’s been around since before the beginning of this particular Marvel Universe.
Knull was born as a god of darkness from the primordial void that filled the gap between the sixth and seventh iterations of the Marvel Multiverse. He was enjoying all that nothingness when the Celestials showed up to spark some life, so one of Knull’s first acts was to forge a blade - All-Black the Necrosword - from his own shadow and use it to behead one of them. That decapitated Celestial skull became the intergalactic space station known as Knowhere. As for the sword, it’s now recognized as the universe’s first symbiote. Later, when he lost the sword, it became the source of power for Gorr the God Butcher, one of Thor’s greatest foes.
Knull started stylizing himself more as a god of symbiotes than a god of darkness from that point on, spreading his gooey, black alien subjects all over the known worlds. But his own creations came to fear his anti-life urges, so the Klyntar imprisoned him within a planet made of their own bodies. And there Knull remained for millennia, doing his best to maintain his influence from within, until he got out and made a beeline for the planet that had had the most trouble with symbiotes: Earth.
Knull wound up going head-to-head with Eddie Brock and Venom, the most notable human-symbiote partnership on record, and in the process, he enslaved a good portion of the world’s superheroes by forcibly bonding symbiotes to them. In the end, however, Venom proved victorious through his strong connection to the Klyntar hive-mind, snuffing out Knull’s existence and replacing him as the King in Black.
Before he departed, however, Knull managed to tie together and intertwine so many different aspects of Marvel history that his impact will continue to be felt for generations to come. As a foil, he also helped elevate Venom from antihero status to true superheroism - and what else is a good villain for?