The Marvel Universe is rife with villains, some of them ancient; some whose existence even stretches back billions of years, but the first villain in the Marvel Universe was none other than Knull, the King in Black. Introduced during Donny Cates and Ryan Stegman’s Venom run, Knull has not only redefined Venom but Marvel villains as a whole.
Over their run on the symbiote hero's series, Cates and Stegman have reshaped the Venom mythos, taking him from a mere antihero to a player in a cosmic saga stretching back to the beginning of time. A key element of this run was the introduction of Knull, the god of the symbiotes. Also referred to as “the King in Black,” he is a resident of the primordial darkness before creation; Knull seeks to return to this darkness by whatever means necessary, be it beheading Celestials or enslaving deities. Among his many powers is the ability to create and manipulate symbiotes from the raw void outside reality - something he does as a step in his plan to destroy existence.
Since Knull’s first appearance, he has been shown to have fought Marvel heroes at various points in the distant past, including the Silver Surfer and Thor. The Surfer, the first hero to engage Knull, did so after being thrown back to the beginning of time. Knull attempted to enslave the Silver Surfer twice, but the Surfer was able to create a star whose light banished Knull, as light is his only weakness.
Thor encountered him during the Dark Ages when one of his symbiote dragons came to Earth and attacked a Viking village. The dragon was christened “Grendel” by the villagers; Thor fought Knull and Grendel and was able to sever the psychic links between Knull and the symbiotes - a crushing defeat. Imprisoned afterward in a cage of symbiotes, Knull waited for centuries, planning revenge.
Recently, Knull was freed from his prison by Carnage, and he was free to begin his crusade again. He tore through the cosmos on his way to Earth and laughed at the heroes of the Earth as they attempted to turn him back, even splitting the Sentry, one of Marvel’s most powerful heroes, in half. There are many ancient villains in the Marvel Universe, such as Ego the Living Planet or Mephisto, but Knull existed before them all, floating in the primordial darkness that existed before the universe came to life. As befits someone so old, he is extremely powerful and difficult to defeat; there is even debate on whether, as an avatar of the primordial abyss, he can even be defeated, or whether he's a natural and necessary part of the galaxy's construction. Regardless, Knull is Marvel’s first, and most deadly, villain, and as the King in Black, one of the biggest threats Earth has ever faced.