Born of Darkness
Knull existed in the darkness when that was all that existed. When light poured into his blackness, he tumbled through existence for eons. During that time, the Celestials found the darkness wanting and began building space with light. After regaining himself, Knull plunged a hand into one of the giant gods and pulled out the proto symbiote which he turned into a sword and used to slay a Celestial. As punishment, the space gods sent Knull back into the void of the murder victim.
Inside, Knull made a home for himself in the head of the Celestial he killed. There, he created the first sword-shaped symbiotes from his own form and developed into the god of the forge. Those earliest symbiotes were far from perfect in those early days and still retain a fear of fire from their violent creation.
With All-Black the Necrosword, Knull set out to kill all beings in the light. In doing so, he crashed on a planet where Gorr, the future God Butcher, made off with his weapon. While stuck on this unnamed planet, the Lord of the Abyss realized that he could create a symbiotic relationship with creatures or hosts. He could not only learn from their knowledge, but also pilot them remotely. On that desolate planet, he realized his true power and began building a hive mind of symbiote agents.
The true extent of Knull's powers remains something of a mystery. He claims to be the god of the abyss, the world of darkness that existed before light entered the cosmos. When the Celestials first made their presence felt, he realized he could make a blade that was able to kill one of the gods. He began forging a new weapon from his own form that he eventually called All-Black, the Necrosword. He used the weapon to fight back agents of light.
During one such battle, he lost All-Black and became marooned on a backwater planet. There he learned that he could bestow an aspect of his essence onto another creature. He could control the resulting symbiotes even across great distances. Altogether, they also formed a hive mind that allowed Knull to become an inter-galactic despot.
An attack on one of Knull's minions by Thor severed his connection to the symbiotes. Eventually, his children became enamored with the light and decided to imprison Knull in a planet-sized planet called Klyntar. Though trapped, he could still used symbiotes to communicate through. Now that he is free, it's unclear exactly how powerful he is, but Eddie Brock and the rest will certainly find out soon.
While Knull's earliest symbiotes have a weakness to sonics and fire, their creator only seems to have problems with light, though the extent of that susceptibility remains to be seen.
Knull ruled the darkness and attempted to harness the bearers of the light known as the Celestials. For a time, he waged a one-god war against those who opposed his darkness. Eventually, he fell onto a nameless planet where Gorr, the future god-butcher stole All-Black. During his time on this orb, Knull began creating symbiotes and sending them out into the vastness of space.
At one point, Knull came to know a time-lost Silver Surfer. Wanting him for his own agent, Knull pursued the skyway surfer who managed to receive help from the nascent Ego the Living Planet before returning to the present.
When Thor attacked one of Knull's dragon-like Grendels, the Mjolnir-born bolt severed all of the symbiotes' connections with their king. Many took on new hosts and embraced the light and came to realize that their former master was evil. They banded together and imprisoned him on a planet they called Klyntar.
More recently, with the Grendel re-awakening on Earth, Knull could communicate through it and came to know Venom and Spider-Man (Miles Morales) as enemies. A cult on Earth dedicated to Knull, the Church of the New Darkness, eventually brought Carnage back to life who acted as an agent for amassing power for the dark lord. Carnage even brought Norman Osborn over to their side, but Venom stopped them with some help from his son Dylan. Still, Knull managed to escape Klyntar where he briefly met Wraith and removed the Exolon from his body before flying off towards Earth.
Knull & Void
Knull's rampage of darkness across existence was carried on from his new throne-world. At some point, a vastly weakened, time-lost Silver Surfer from the present landed on Knull's world and created a new sun with the Power Cosmic to fell the warriors he encountered. Victorious, the displaced Surfer entered the nearby stronghold only to come face-to-face with Knull! The dark lord managed to place one of his symbiotes on the Surfer, but the traveler received assistance from Ego, which allowed him to regain his freedom. Enjoying the experience, Knull gave chase.
Knull caught up to Silver Surfer who was being overtaken by the growing symbiote darkness. Though Knull proved more powerful, the Surfer managed to spread life and light to the cosmos which eventually allowed him to be remade back in his present. This act of defiance greatly displeased Knull.
When Knull's minions arrived on Earth, they came upon a force unlike any that had faced before: Thor. When the God of Thunder attacked one of Knull's dragons with a bolt from mighty Mjolnir, it severed the pan-galactic shared mind, leaving his agents -- including the dragon-like Grendel -- adrift. At this point, the symbiotes sought out new hosts, but these new unions allowed darkness in where previously there had only been darkness. With this new knowledge, they rebelled against their creator and eventually cast him into a new abyss, using their forms to build a planet-sized prison around their former master.
At this point, they became the more noble Klyntar. During his first tenure as Herald, Silver Surfer investigated this planet and had no idea why his presence caused such chaos amongst the symbiotes and their imprisoned creator! The symbiotes called this new place Klyntar which translates into "cage," in their language. Knull remained there until recently.
Gone, But Not Forgotten
When Knull sent the Grendel to Earth, it inspired a cannibal cult to worship this monstrosity. After it was felled by Thor, pieces of the symbiote fell to Earth and bonded with members of the cult, later to be known as the Church of the New Darkness, who eventually made their way to the new world. The group put down roots in the woods around what is now known as Westchester County, New York.
In the 1660s, Cortland Kasady had been possessed by the followers of Knull on Earth. When he killed several of his fellow villagers, a prison was built around the cave where he came under their influence. His wife, Molly Ravencroft, came to visit him and he used her to escape, killing Molly in the process. Molly's descendant Jonas Ravencroft would go on to found the asylum that would hold many of the world's most maniacal killers, including Carnage.
The Grendel that the Church of the New Darkness worshipped fell to Earth and froze in the Scandinavian tundra. In 1966, S.H.I.E.L.D. dug it up and took possession under the watchful eye of Nick Fury. Because the powers-that-be still wanted a new Super Soldier, the scientists bonded parts of the Grendel to five different soldiers, dubbed Sym-Soldiers, lead by Rex Strickland. When they went A.W.O.L. in Vietnam, Fury sent in Logan along with an L.M.D. of himself to bring them in. Only Strickland came back coherent while the others were put in cryo-freeze. S.H.I.E.L.D. even brought Strickland in as a symbiote expert not knowing he was still secretly connected to his own!
Re-Enter the Dragon
In the present, Rex Strickland forced a meeting with Eddie Brock and told him about his old buddies who were being moved by S.H.I.E.L.D. When Venom agreed to help free them, he inadvertently unleashed the Grendel dragon!
After Eddie Brock and the Venom symbiote accidentally unleashed the Grendel on Earth, he became aware of Knull. By taking control of the dragon's body, Knull defeated Spider-Man (Miles Morales) and introduced himself to Brock. After separating human from symbiote, the Lord of the Abyss attempted to purge the light from the Venom creature, but ultimately failed.
While still imprisoned inside Klyntar, Knull explained much of his history to Brock and his symbiote. He also warned them that, after the now-freed Grendel found all of it's missing parts, it would return to Klyntar and free him! Brock realized that Rex was actually all symbiote and, as one of the remaining pieces of the Grendel, offered to absorb him into his own symbiote. Brock, Venom and Rex then worked together to kill the Grendel with fire, exploiting one of its few weaknesses and breaking its connection to Knull.
Seeking to resurrect Cletus Kasady and Carnage so they could help bring about the return of Knull, the Church of the New Darkness stole a Codex discovered by The Maker while he was examining symbiotes. They injected Cletus' body with the Codex and it brought him back. True to form, Carnage began killing immediately, but did get an important information from a cult member: he could amass more power by absorbing all the Codicies from other symbiote hosts.
After several other adventures and participating in the War of the Realms, Brock finally got word about Carnage's return and his pattern of tearing through former symbiote hosts. To further Knull's mission, Carnage employed a variety of maniacs from the garden variety to big time names like Norman Osborn. Meanwhile, Eddie allied himself with Spider-Man (Peter Parker) who had also once bonded with the same symbiote.
Giving Up the Ghost
On the War Path
Though he had his sights set on Earth, it took the Lord of the Abyss a while to get there. Along the way he stopped to ravage the planet Fallo. On Earth, Dylan secretly kept a piece of Norman Osborn's symbiote which originally came from the Grendel. Through this, Knull and Dylan have been in contact, including the boy witnessing what the dark god did to Fallo.
Meanwhile, Brock did his best to purge himself of the Carnage symbiote which had bonded with him. After that ordeal, he went to the Avengers and told them of Knull's looming threat, which Thor took very seriously, remembering the menace from his past. Now the Earth and its heroes can only wait as Knull slices a swath of destruction on his way to them in hopes of gaining more power and plunging existence back into darkness.