The Man in Black, the symbiote god in Marvel Comics, was introduced in Venom, but the X-Men had a Man in Black long before Knull.
The Man in Black is an older god and father of the symbiote (or Klyntar) in the Marvel Universe who first appeared in the Venom comics — but unbeknownst to many fans, the X-Men already have their own Version Long before Knull's debut, "The Man in Black" was released.
Knull aka the King in Black debuts in Venom #3 with Donny Cates and Ryan Stegman, his full backstory is described in the next installment. As revealed in Venom, Knull existed before Light existed in the universe, and he alone rules the Eternal Void. When the universe was created and light and life began to flood his kingdom, Knur responded by shaping the Void into a race of sentient beings: the Krynthal. Knull's first use of his symbiote was forging his all-black Necromancer sword, which he used to decapitate Celestials and murder many other Marvel gods. Eventually, the Black Lord created enough Klintas to raise an army, which he sent across the universe, uniting the minds and bodies of everyone they possessed into his endless void.
In Fantastic X-Men #2 Directed by Fabian Nicieza and Andy Kubert, the X-Men continue their mission to free humans and mutants from an apocalyptic prison known as The Pin throughout the villain realm. The series takes place in the Age of Apocalypse timeline, which means Apocalypse has basically conquered the world and imprisoned anyone who stands against him with the help of his loyal followers. One of the most effective Knights of the Apocalypse is a villain known as Abyss - a mutant with particularly insidious powers that make him resemble the Lord in Black himself.
Age of Apocalypse’s Abyss is the X-Men’s Version of the King in Black
Like the God of the Abyss, the Black King, the Abyss is a dark being capable of becoming and moving in shadows. Abyss can stretch, detach, and twist his body like a symbiote, though he's not connected to Klyntar in any known ability. However, the most similarity between Abyss and Knull isn't in their looks or abilities, but in their respective connections to the Void. The Abyss can enter the void, and its entry point is actually inside his chest cavity. In this installment, readers witness the abyss stalking in the shadows of this ruined world, surrounding people (in a For example, a child), and throw them into the void through the portal in his chest.
The Black Lord is the God of the Abyss and the ruler of the Void in the Venom series, and for all intents and purposes, the Abyss as well. The Knight of the Apocalypse has demonstrated a mastery of the void that rivals even Knull - plus, Abyss was the first man he made his Marvel Comics debut in the '90s, while Knull wasn't introduced until 2018. So, from his powers and the fact that he was introduced in the first place, it seems clear that the X-Men had their own "King in Black" long before Venom.