Long ago, before the existence of the universe, there was a being named Knull. Knull was a dark God, an ancient and evil tyrant who lived in a slumber in the timeless abyss before the existence of time. He awoke and retaliated when the Celestials started the creation of the universe by forging a sword from the Living Darkness called the All-Black. Knull used the All-Black to kill one of the Celestials, and using the celestial energy from the murdered Celestial, Knull tempered his sword to become more powerful and manifested a symbiotic armor.
With his ability to create symbiotes, Knull helped a group of baby rodentoid aliens to survive by making them bond with symbiotes that he created, turning them into predators. The symbiotes created were also then used to conquer planets and consume entire civilizations. When the symbiotes finally escape the rule of Knull, they formed a nation called Klyntar subsequently naming their native planet, Planet Klyntar. Upon escaping their bond, the symbiotes had all memory wiped out because they were created as such. In the 998th generation of the Klyntars, a symbiote was hatched but was for some reason neglected by its species. That symbiote was discovered by Kree explorers who brought it back home to study it. The symbiote was eventually bonded with a Kree soldier before undergoing training to fight in the Kree-Skrull War. The symbiote was separated from its host in an attempt to rescue Kree survivors, where it crash landed with them on an unknown icy world.
On that planet, the Symbiote bonded with an arms dealer named Haze Mancer, who was set on destroying his own world. After separating itself from Haze and losing its memory again, the symbiote was captured by the Symbiote Imperial and imprisoned. It was only much later that the symbiote would first bond with a human, and that human was Deadpool. Afraid that the symbiote would be influenced by his insanity, Deadpool separated himself from it and returned it to its prison, only for it to be discovered by Spider-Man moments later. Spider-Man had suffered damages to his suit, so upon discovering the symbiote, he bonded with it to form the first ever Spidey symbiote combo. The symbiote augmented Spider-Man's abilities and quickly manifested his powers for itself. Having a mind of its own, it would wait for Spidey to fall asleep before going into action at night.
It was much later and after many disputes between Spidey and the symbiote that they would be properly separated with the help of the Fantastic Four, and that it would develop a bitter feeling towards Spider-Man. Their love-hate relationship is what makes it eventually bond with Eddie Brock to become VENOM.