In the waning days of the 20th century, the ancient mutant called Apocalypse continued to do what he had done for over five thousand years: survive. However, while his strength was prodigious and his powers nigh unstoppable, the preceding millennia had caused Apocalypse to burn out his body, forcing him to regenerate during periods of hibernation and, at times, literally robbing the strength of others who were among "the strong." Sensing Nathan Christopher Charles Summers, the child of Scott and Madelyne Summers, to be one such mutant, Apocalypse directed his recently acquired followers, the Riders of the Storm, to kidnap the infant and bring him to Apocalypse's fortress in the "blue area" of the Moon.
In short order, the assembled team of the original X-Factor followed to rescue Nathan and were joined by the Inhumans, who wished the forces of Apocalypse off of their territory. Though they succeeded in defeating Apocalypse, the damage was done and young Nathan had been infected with a techno-organic virus Apocalypse had obtained from his Celestial ship. Fortunately, aid arrived in the form of a woman calling herself Askani, who claimed to be having been sent to save "the Chosen." Faced with the choice of either imminent & definite death of his son or giving him to a stranger who might be able to save him, Cyclops chanced on the latter and gave Nathan to Askani, who disappeared into the beyond. [X-Factor (1st series) #65-68]
What Askani had withheld from Cyclops, no doubt due to its utter implausibility, was that she was from the far future, nearly two thousand years hence in the 38th century. More surprisingly for Cyclops would have been that Askani was actually but one of a group of mutants of the same name, led by a future version of Cyclops' other child, Rachel Summers. Known as Mother Askani in this era, Rachel had sent Askani back in time to save her brother from Apocalypse, believing him to be the Chosen One who would save the world from the Apocalypse of that era.
Using their medical technology of the day, Mother Askani successfully halted the spread of the virus in her brother but, as a precaution, ordered the creation of a clone in case the treatment proved to be a failure. The procedure of the medical cyborg Boak was a success and the clone was healthier by far than its cellular template, with the scars of the techno-organic virus barely manifesting themselves. More surprisingly, however, the original, infected infant, Nathan himself, did not die as expected.
Unfortunately for the Askani, they had no time to rejoice in their good fortune, as the arrival of the infant Nathan had not gone undetected. Apocalypse himself had sensed the infant mutant of true potential and ordered his troops of his Canaanite regime to attack the Askani base. Mother Askani and her clan did their best to evacuate their sanctum but Boak was only able to recover one of the two infants, the original Nathan. Left behind, the clone was recovered and claimed by a jubilant Apocalypse, who welcomed the child to a "world of Stryfe." [Cable (1st series) #7-8]
Completely unaware that the child was a clone or that its original template lived on elsewhere, Apocalypse brought the infant back to his seat of power. He named the child Stryfe, after an ancient enemy of his, and then turned him over to the Paladin Ch'Vayre to raise. Though overall Ch'Vayre was honorable in his own way and tried to instruct the boy as well as he could, Apocalypse ordered the growing youth's education at the hands of his dog soldiers, whose abusive training and emotional and psychological torture hardened the child. Under their tutelage, the young Stryfe became prone to violence and murder, a fact made worse as Apocalypse worked to accelerate the youth's mutant powers. As a result of his ever-increasing power and violent tendencies, Stryfe earned the moniker of "Chaos-Bringer."
To Apocalypse, Stryfe was perfect - the personification of the tenet of survival of the fittest and everything that Apocalypse himself ever wanted to be. 
In this, Apocalypse held close a secret. As far as the world (and Stryfe himself) knew, Apocalypse was preparing the youth to be his heir. However, in reality, Apocalypse intended Stryfe to become a host to sustain his essence, an act Apocalypse had tried to accomplish almost two thousand years before with Nathan, whom Apocalypse believed this child to be. Having already needed periodic regeneration two thousand years before, in the two millennia since Apocalypse's situation had grown worse and he had been forced to take a new form every few years and spent less and less time in each new manifestation, as if burning them out. He'd been looking for a frame that could accommodate his power and he believed he had found it in Stryfe.
After more than a decade of preparation, the day arrived when Apocalypse prepared for the transfer of his essence to Stryfe's body. To Apocalypse's surprise, he found himself betrayed by Ch'Vayre, who strongly disagreed with Apocalypse's violation of his own tenet of survival of the fittest by this action. To do so, he aided in the infiltration of Apocalypse's citadel by the rebels Slym and Redd, who were in actuality the mentally and temporally displaced Scott and Jean Summers, who had been brought into the future and placed into cloned bodies by Mother Askani to raise Nathan.
Still, their interference was not enough to stop Apocalypse, who prepared to finalize the transfer. However, in his moment of victory, Apocalypse realized to his horror that Stryfe was not a true lifeform but a gene-copy and thus an incompatible host. Now at his weakest point, Apocalypse was helpless under the combined attack of Slym, Redd, Ch'Vayre and the young Nathan. Unfortunately, just as Apocalypse succumbed to death, so too did Rachel, ending her decades as Mother Askani and her time maintaining Scott & Jean's connection to their cloned bodies. As she expired, they disappeared, leaving Nathan Dayspring alone. [Adventures of Cyclops and Phoenix #1-4]
Knowing that the death of Apocalypse would not be the end, that one of En Sabah Nur's followers would wish to inflict harm on the young boy responsible for the fall of the dynasty of Apocalypse, Ch'Vayre fled with the unconscious Stryfe, hoping to raise the youth into a less cruel adult. Unfortunately, as the years passed, Ch'Vayre met with little to no success, as Stryfe remained convinced that, as the sole heir to the throne of Apocalypse, it was his destiny to rebuild his kingdom. Worse for Ch'Vayre, the years had sapped his will, leaving him subservient to Stryfe's wishes.
Gathering tools for this task of ascension of power, Stryfe found and reactivated an android ADAM unit, designated "Zero." With the combined assistance of Ch'Vayre and Zero, Stryfe traveled the globe, attempting to gain control over whatever was left of Apocalypse's weapons. When he could not find any functioning remnant technology, Stryfe embarked on a different plan. Having been made aware that one of the old Askani order, Madame Sanctity, lived, Stryfe set out to learn to further his own powers. On the way, however, Stryfe detected thoughts of betrayal from Ch'Vayre, who feared a more powerful Stryfe. Enraged by the latest betrayal, Stryfe killed the man who had been a mentor his whole life. [Askani'Son #1-4]
Eventually, Stryfe found Madame Sanctity and saved her just as New Canaanites forces struck. 
Already a woman of tenuous sanity, Madame Sanctity was intrigued that her savior possessed the same psi-signature as the "Chosen One" and accepted his offer of an alliance. With the world believing her dead from the New Canaanite attack, the two departed to parts unknown. It's not clear what happened to Madame Sanctity after this, as she was never heard from again. However, as Stryfe's powers indeed grew in scope and control, it is no doubt that Stryfe greatly benefited from Madame Sanctity's tutelage. What is unclear, however, is if Madame Sanctity herself benefited from her alliance with the Chaos Bringer. [Askani'Son #4]
The years that followed were a period of consolidation and preparation by various parties across the planet. For the High Lords of New Canaan, they continued to solidify their control over the whole planet, defeating the remaining nation states that opposed them. However, as territory after territory fell, in their place rose small packets of resistance, one being the so-called Clan Chosen, led by Nathan Dayspring, who now went by the name of Cable. Their goal was the eventual overthrow of the Canaanite rule. Around his twenty-fifth year, Stryfe too rose as a force against the global hegemony. However, his goals were much less altruistic, as he intended to supplant, rather than overthrow, the planetary dictatorship. [Cable (1st series) #1]
Hidden behind his carefully forged armor, which doubly served as both protection and to bury his human soul within a cold, metal shell, Stryfe became a self-proclaimed anarchist and fomenter of political dissent. Along with his followers, whom he called Scions of the High Lord, he participated in acts of aggression against both the High Lords of New Canaan as well as rebellious forces such as Clan Chosen. During his reign of terror, he leveled cities and firebombed forests, leaving rivers of carnage in his wake. In short, he annihilated everything that ever got in his way. [X-Force (1st series) #74]
During this time, Cable was completely oblivious to the connection between himself and the Chaos Bringer. However, it is unclear if the same could be said of Stryfe, who clearly knew long before Cable was made aware. It could be that he was so informed by Madame Sanctity during their time together, thus giving reason as to his interest in someone whose efforts went against a powerful, mutual enemy. It is also likely that Stryfe merely saw photos of Cable and recognized Cable's face as his own. Though Stryfe continually operated behind an ornate helmet, Cable's visage was well known.
Whatever the motive, Stryfe undertook an operation attacking Cable's Clan Chosen in which Cable's wife, Jenskot, was killed and his son, Tyler, was kidnapped. Seeking both revenge and recovery of their kinsman, Clan Chosen tracked Stryfe to the old, abandoned venting tunnels below Lake Michigan. There they found Stryfe and his forces, as well as a brainwashed Tyler. When it became clear to Cable that Tyler was beyond saving and a clear threat to their escape, Cable felt he had no choice and fired upon Tyler, killing him. [Cable (1st series) #1, #25]
For some time, Cable had gained access to time-sliding technology and had made periodic jaunts into the past to learn more about the eventual ascension of the High Lords in his and Clan Chosen's quest to overthrow them. However, when the New Canaanites defeated the remnants of global resistance in late 3806 and thus consolidated their control over the planet, Cable realized that he and Clan Chosen had failed. As such, Cable decided upon a different plan and traveled back in time to the 20th century to alter time itself. [Cable (1st series) #minus1] Likewise seeing no way in which he could usurp control of the planet, Stryfe decided to follow. While Cable intended to control the ascension of the High Lords from its inception, Stryfe intended to prevent it and along the way sow chaos by exacerbating the human-mutant relations in its early years. [X-Force (1st series) #17]