Together with Unit Zero, Stryfe arrive in the late 20th century and immediately began forming his secret empire. In the early years of his time in the era, Stryfe seemed to lay low, slowly consolidating resources and funds, such as with arms deals. During one such deal gone bad, Stryfe murdered Louis Alejandro Garabello Richter, head of a Mexican gun-smuggling syndicate and father to Julio Esteban Richter, the future mutant known
as Rictor. As Stryfe did so while without his trademark helmet, Rictor memorized the face of his father's murderer and would remember it years later when he encountered Cable. [X-Force (1st series) #34]
Another source of income for Stryfe's burgeoning organization was with the mysterious Tolliver, who hired Stryfe to maintain an opium route through Soviet-occupied Afghanistan. For reasons of his own, Tolliver wished to set Stryfe against Cable, who at the time had been operating with a group called the Six-Pack as mercenaries for hire. On a mission for Tolliver in Afghanistan's Khyber Pass, the Six-Pack encountered Soviet heavy armament searching for what the Six-Pack quickly learned was a secret installation. Curious, Cable investigated and was immediately shocked when he recognized the facility was constructed with technology from his era. Of even greater shock was the sight of Stryfe himself, who was also surprised, though unfazed, by the appearance of his old foe.
Horrified at the idea of the Chaos Bringer being in this era, Cable unleashed a charge of high powered explosives, destroying the facility. Unfortunately for Cable, Stryfe escaped via Zero's teleportation portal, moments before Cable body-slid himself and the rest of Six-Pack to safety. Refusing to give up, Cable searched for Stryfe over the next year, dragging along the confused Six-Pack and refusing to answer their questions. Cable's search ultimately led them to the mountains of Uruguay, the location of another of Stryfe's base. Their initial plan was to steal data from the base's computers and blow up the facility before Stryfe was aware they had arrived. However, as they prepared to depart, Stryfe appeared and recovered the data disc, mocking that their bodyslide technology would not work, preventing their escape from a place lined with their explosives, which were set to detonate. As Stryfe escaped once again via Zero, Cable side-stepped the bodyslide countermeasure by timesliding back to the future. Abandoned by their leader, the rest of the Six-Pack were left in a self-destructing facility, a betrayal they would never forget. [Cable: Blood & Metal #1-2]
As Stryfe's criminal actions in the early years after his arrival in the 20th century would hardly be worthy of one called "the Chaos Bringer," it seems clear to say that they were attempts to build capital and resources for his later actions. As these years coincided with early years of mutant emergence, it's likely that it would have been useless to recruit a mutant population that did not yet exist in great numbers. Additionally, as Apocalypse, one of Stryfe's greatest targets of revenge in this era, had not yet made his presence known in the modern world, perhaps Stryfe awaited his return to begin his own reign of chaos.
In preparation, however, Stryfe formed the Mutant Liberation Front, colloquially known as the "MLF." While Stryfe had employed humans in his service so far, the members of the MLF were all mutants. Upon this persecuted minority, Stryfe promised that together they would together use the fear their powers could generate to force humans to respect them. In their first public act of terrorism, the MLF destroyed an energy research facility and warned that other such acts would continue until the recently-captured mutants Rusty and Skids were released. When it became clear that authorities would not comply, Stryfe ordered the MLF to rescue the two youths themselves. Despite the attempted interference of Cable, who it seems already was aware of the MLF and its members, the MLF succeeded and teleported Rusty and Skids back to their HQ. [New Mutants (1st series) #86-87]
Though through his disciples Stryfe had indeed gained a minor victory over his old foe, Stryfe had inadvertently brought Cable into the lives of the New Mutants, who for months had been without a leader. In Stryfe's oldest nemesis, the New Mutants found direction and in them Cable had found a tool to use against Stryfe himself. Stryfe learned this to his regret some months later when Cable and the New Mutants tracked him to the island nation of Madripoor. There they learned from Sunfire that Stryfe had tested a deadly drug in rural Japan and now intended to introduce it other major cities, starting with Madripoor. Once gripped with fear, the rest of the world would succumb to his demands. Before his plan could come to fruition, however, Cable and the New Mutants (with the help of Wolverine) destroyed the drug before Stryfe could deploy it. [New Mutants (1st series) #93-94]
Though Stryfe maintained loyalty from the soldiers in the Mutant Liberation Front through a common cause, he nonetheless yearned for a larger army than he had been able to recruit. To this end, Stryfe enlisted the aid of contemporary scientists who could genetically engineer his next generation of mutant disciples. Gathering a group of likeminded scientists, Stryfe secretly gained control over a project called Stepladder, whose goal it was to engineer the next step in man's evolution and which had been experimenting upon the Apache tribe at Camp Verde. However, when some of the researchers attempted to let the truth out, Stryfe ordered the entire population of Camp Verde massacred to maintain the secret. Knowing such an act would draw attention, possibly Cable's or Xavier's, Stryfe planted a mask of an agent of the Hellfire Club to throw them off his own trail.
Unfortunately for Stryfe, the plan worked all too well, as James Proudstar, known as Warpath, was thus easily recruited by Cable into his new X-Force, which Cable had formed from what remained of the New Mutants. After months of training them into an effective combat squad, Cable led the new team to Stryfe's latest base of operations in Antarctica. With bloody efficiency, X-Force made short work of the MLF forces there, forcing Stryfe to abandon the facility and destroy it in his wake. [New Mutants (1st series) #99, X-Force (1st series) #1]
Despite more setbacks, the MLF continued its operations. When one of its members, Thumbelina, learned that her brother, the mutant Nasty Boy called Slab, had been captured and was being held in government custody, Stryfe sent the MLF to rescue Slab and his teammate, Hairball. Despite the intervention of the government sanctioned X-Factor, the MLF succeeded and the two Nasty Boys were rescued. Shortly thereafter, the master of the Nasty Boys, Mister Sinister, arrived to claim his lost minion. [X-Factor (1st series) #77-78]
It's not clear who contacted whom between Sinister and Stryfe, or how much each knew of the operations of the other, however the meeting of the two must have been a pivotal moment for Stryfe and his plans in this era. Due to their dealings that soon followed, it can be surmised that Stryfe learned of Sinister's connection to Apocalypse and of Sinister's interest in the lineage of the Summers family. It is very possible that from this meeting Stryfe learned of Sinister's involvement in the conception of the child between Scott Summers and Madelyne Pryor, the result of which was sent to the future. If he did not know of his true parentage before, during this meeting Stryfe learned of it and the circumstances leading to his adoption by Apocalypse. Soon after this meeting, Stryfe and Sinister would strike a deal which would give Stryfe what he wanted most of all: revenge.
Possibly heartened by this turn of events, Stryfe allowed a crucial piece of information to make its way back to his old foe, Cable. When Garrison Kane, the so-called Weapon X and an embittered former member of Cable's Six Pack, followed fleeing members of the MLF through one of Zero's portals, he came literally face-to-face with Stryfe. After years of hiding his true visage to those who might know Cable, Stryfe removed his helmet and mocked Kane's confusion at how Cable and Stryfe could possibly be the same person. Leaving him with nothing but cryptic questions, Stryfe knocked Kane out and abandoned him a short distance from his home. [X-Force (1st series) #9-10]
Stryfe's actions with Kane might have been an act of calculation, rather than madness. As an enraged Kane helped form Weapon: Prime to take down Cable and X-Force, Stryfe had cover to scour the globe for his master plan. So loyal were the members of Stryfe's Mutant Liberation Front that they did not question their master's orders to raid museums to acquire specific artifacts seemingly from random parts of the world and random eras of history. 
Having survived his encounter with Weapon: Prime and eventually convincing Kane that he was innocent, Cable tracked Stryfe's operations to a remote area of Mexico. Unfortunately for Cable and Kane, their arrival had been anticipated by Stryfe, who set a trap. With both at his mercy, Stryfe again gloated cryptically regarding making those who had robbed him pay, as well as explaining that the common thread behind the artifacts had to do with their mutual enemy: Apocalypse. Before he could deliver his coup de grace, however, final victory was denied Stryfe when Kane fought back, allowing Cable an opportunity to timeslide the two to safety. [Cable: Blood & Metal #1-2]
Though he still lived, Cable was nonetheless out of the picture for Stryfe, who took the opportunity to enact his master plan of revenge. Using his likeness to Cable to his advantage, Stryfe disguised himself as Cable and shot Professor Xavier during a speech at a Lila Cheney concert. While the X-Men, X-Factor and X-Force reacted in horror and confusion, Stryfe enlisted the aid of his new ally, Mister Sinister, for the next part. Disguising himself as Apocalypse, Sinister ordered the Horsemen of Apocalypse to capture Scott Summers and Jean Grey, at which point Sinister traded them to Stryfe in exchange for a canister supposedly containing genetic material of the Summers line for the next two thousand years.
In two strokes, Stryfe had neutralized Xavier and left his followers in disarray, as well as captured the parents who had abandoned him. Still, the ramifications were not complete, as Apocalypse other minions, the Dark Riders, were all too aware that their master could not have ordered his Horsemen to capture Scott and Jean, as Apocalypse was still in the middle of a regeneration cycle in remote Egypt. 
Knowing that this could not wait, the Riders awoke their master, who desperately attempted to determine who had posed as him, bringing him in direct conflict with the X-Men. Already weak from having been awoken too soon, Apocalypse returned to his Egyptian base for another attempt at regeneration. It was at this critical point that Stryfe attacked, personally besting the Dark Riders and, before their eyes, defeating their master, who was forced to flee. Having witnessed their master defeated by someone stronger, the Dark Riders stayed true to their creed and pledged loyalty to the strongest: Stryfe.
His ranks bolstered by his new followers, Styfe returned to his current base of operations, the Blue Area of the Moon, where Apocalypse had taken infant Nathan Summers so long ago. There he continued to torture Scott and Jean, treating them as if they were infants and accosting them for acts of abandonment which they did not comprehend. In an ultimate Machiavellian attempt at reenacting the circumstances of their past sins, Stryfe allowed Scott and Jean to escape their cells so that they would be faced with a horrific choice. During their escape attempt, the two X-Men came across an infant that had been cybernetically connected to the facility and were informed by Stryfe that all they had to do to escape and destroy him was to kill the child. To his shock, Stryfe witnessed as the two refused and ultimately were recaptured.
As he attempted to contemplate Scott and Jean's actions, which were at odds with what he had previously believed, Stryfe began to see that his ornate game of chess had become undone. The chaos created by Xavier's assassination attempt had run its course, with all three X-teams finally uniting in their investigation. As bad as their realization that he, Stryfe, was behind it all, the group had deduced his location and allied with Apocalypse himself, who possessed the means to transport them all to the Moon. Preparing for their arrival, Stryfe constructed a tower which generated a time-vortex forcefield through which only those with Summers DNA could pass. As expected, Cable succeeded in gaining entrance and Stryfe reveled in using his telekinesis to toss around his oldest foe like a ragdoll. Giddy with power, Stryfe finally revealed what he had long hinted at: he, Stryfe, was the son of Cyclops that had been taken into the future and that Cable was his imperfect clone.
Though seemingly on the cusp of his long-sought revenge, Stryfe had not anticipated that Havok as well would be able to traverse the forcefield. He was thus was caught off guard by Havok's plasma blasts, allowing Cyclops and Jean to gain their freedom. As Stryfe raged, directing his telekinetic fury at Cyclops, Jean and Havok, Cable moved to end the insanity. Activating a self-destruct mechanism in his mechanical arm, Cable tackled Stryfe at which point he activated a temporal portal that swallowed them both. In the wake of the resulting explosion, the X-Men could only surmise that the two were killed. [X-Cutioner's Song part 12, X-Force (1st series) #18]
Though defeated and apparently dead, the last piece of Stryfe's revenge survived him. In a hidden facility in Nebraska, Mister Sinister prepared to enjoy his prize, the canister containing the Summers family genetic materials. To this disgust, however, Sinister soon realized that he had been cheated when his scientist found the canister empty. Unknown to Sinister or his assistant, the canister had indeed contained something, albeit a substance undetectable to the naked eye. When the canister opened, an airborne virus brought back from the future by Stryfe was released. Originally engineered by Apocalypse's biologists two thousand years in the future to wipe out the remaining non-mutants, the virus had never been completed. Now, in the late 20th century, Stryfe had completed it, altering its design to attack mutantkind. Over the next few years, the deadly so-called Legacy Virus would infect and kill untold number of mutants until a cure was found. [Adventures of Cyclops and Phoenix #3, X-Force (1st series) #18, Uncanny X-Men #390]