Knull made his first appearance in Thor: God of Thunder #6
Knull has been around since the dawn of the Marvel Universe. As a matter of fact, he lived when there was nothing. Then the Celestials came and shaped the universe, making 'space' as we call it. Knull wasn't fond of this, as this was apparently his domain, or as he calls it, his kingdom. One would question this, since it was all empty anyways, however, he claims that his kingdom, his abyss, was never empty and dead.
Furthermore, he creates a living sword from his shadow. With this sword, he decapitates a Celestial. The other Celestials quickly banished him to the void. This was, however, not a smart idea because the void is where Knull thrives.
He forges a sword, the first race of the symbiotes. His living blade would be known as the All-Black/Necrosword/Godslayer.
Knull realises that he's all on his own. Knull bonds his living abyss with lesser creatures, therefore he's able to draw strength from them and control them. He's now their God, or rather, he's become their God-Host. He can see through their eyes and control them, even from across galaxies. These creatures would later become the alien race known as symbiotes. Knull would also create a planet to act as a throne-world for his conquests- what made it easier was that the planet itself was created out of his creatures, allowing him to corrupt any being that dared to enter his domain.
Knull's army of symbiotes travelled far and wide, destroying countless planets and stars in the process. The more they took over, the more Knull found out about the universe- each host corrupted by his new creatures had their mind completely accessible by him, so he was able to learn more and more about how the universe functioned, and how the light had taken over most of the Abyss that he once owned.
During this age of conquest, a time displaced Sliver Surfer found himself in this time period, and was attacked by Knull. His powers weakened, Surfer was unable to fight back against him, and Knull attempted to use a symbiote to control Surfer as well. However, a infant Ego, sensing the pain of Surfer, managed to save him with a blast of heat, which kept the symbiote at bay. Knull attempted to catch Norrin using one of his symbiote-dragons, but he was able to outrun it. Ego later nursed him back to decent health, and they fought again, this time with Ego assisting him- however, Norrin admitted that Knull was far too powerful to be destroyed, and used the very last of his Power Cosmic to send Knull into a sun he himself created, and as a result sent Knull blasting through the cosmos. While this slowed him down however, it did not kill the warlord, and he continued his war against the light and the beings who embraced it.
During a battle with multiple gods, he crashed unto a barren and seemingly lifeless planet, where he was subdued and knocked out. During this, a weak and frail alien known by Gorr, who at that moment had been praying for his death as he was hungry and had been banished by his tribe, noticed the battle and where they had fallen, and managed to gain control over the All-Black itself, and killed one of the gods who had been injured by Knull's attack in the process, vowing revenge on all of the gods for their refusal to save him or his son.
While Knull eventually recovered from this attack, he was never able to recover his blade back again: a event that arguably was a grim sign of things to come.
During the sixth century, one of Knull's creatures made it to the planet known as Earth , and attempted to conquer it. The natives at the time dubbed it the Grendel- a creature of darkness itself. However, this creature had the unfortunate fate of encountering the mighty Thor , and they battled, with Thor hitting the creature with a massive divine lightning bolt. This blast was so violently painful that the impact severed the connection between Knull and most of his symbiotes, leaving them without a master controlling their thoughts and feelings. Having bonded to their hosts and established a emotional connection to them, many of Knull's creatures were consumed by the idea of honor and respect, pushing the hive mind to turn against their god by trapping him within the planet of his own making. Unable to control them anymore, Knull was imprisoned within a living prison of his once loyal creatures- trapping him deep within their planet, now dubbed Klyntar, which was their word for "prison" conistently shifting around to ensure that Knull would never be able to break out again.
Ashamed by their actions while under the heel of their creator, Knull was removed from the minds of many symbiotes and they in turn established a new order for themselves, in which they'd be able to govern without a master controlling them. Knull vowed revenge on his traitorous children for what he considered their betrayal.