Bishop - a Literal Cassandra ?
Many months before the X-Men ever heard of the being known as Onslaught, in fact even before the creature was birthed in the first place, there was a warning of things to come. Right upon joining the X-Men, did the time-displaced Bishop tell his new teammates of an event that had transpired in the future timeline he came from. While hunting down a criminal, he and his fellow X.S.E. officers accidentally happened upon a video recording from long ago made by a terrified Jean Grey. 
Most of the message was scrambled, but from the few parts that remained intact Bishop learned about there having been a traitor among the X-Men, who killed them all when they were unaware, and even seemed to have gotten to Jean by the end of the tape. The mutants didn’t know what to make of Bishop’s story - they barely knew the man and his wild accusations towards Gambit, based upon seemingly recognizing him as the Witness, an elder man from his timeline who was reported to have been the last person to see the X-Men alive, didn’t help much either. Bishop had to eventually agree that even if Gambit was to become the traitor he believed him to be, they couldn’t punish him for something that he hadn’t done yet. [Uncanny X-Men #287, X-Men (2nd series) #8] Life went on, and the X-Men averted one attack after the other. Along the way, they began to trust Bishop and accept him as one of their own. Bishop himself sure remembered the disturbing message he had seen, but there wasn’t much he could do about it anyway, other than keeping his guard up. However, after a while he wondered if the future really was a pre-destined as he thought it to be, and he even began to develop some sort of working relationship, if not a friendship, with Gambit himself.
Who or what is “Onslaught“ ?
Time passed. Eventually the X-Men had some downtime, and some of the team went to New York to spend an evening in town. Naturally, their night out was interrupted by them being called into action again, some rather disturbing matter. In Hoboken, New Jersey, they found the crashed down form of the Juggernaut. When he came around and learned where he was, the self-proclaimed unstoppable villain couldn’t believe it. Actually he had been in Canada when he was attacked – someone had knocked him half across an entire continent! Still dazed and confused, Cain Marko was frightened to the bone and could only repeat his attackers’ name, Onslaught, before passing out again. Although Bishop was among the X-Men who found the beaten Juggernaut, not even he linked this incident to the traitor tape he had seen; at the time nobody could have known that one of the X-Men’s own hid behind the alias of Onslaught. As it turned out there was no chance to further interrogate the Juggernaut of the mysterious being, as he broke out of the mansion’s infirmary soon thereafter, still panicking that Onslaught would come after him. He was then teleported away to a side-realm by two operatives of a secret agency, who seemed to have a special interest in nobody learning Onslaught’s identity too soon. [Uncanny X-Men #322, Wolverine (2nd series) #93]
The X-Men were not the only ones puzzled by the mystery regarding Onslaught. When governmental agents checked on a High Security Sentinel Augmentation Research facility in Kentucky, considered humanity’s first line of defense against the mutant threat, they found the place abandoned - fifteen of the United States‘ most highly regarded scientists missing. The base’s computer systems went back online shortly afterwards, but only a single word appeared on the screens: “Onslaught.“ [X-Men (2nd series) #46] An investigation of Project Nimrod at Camp Hayden brought up the same result; X-Force found the base stripped bare of all it’s devices and computers, the word “Onslaught” painted on the walls instead. Although it was still unclear who or what Onslaught was, it was obvious that the entity’s plans involved Sentinel technology. [X-Force (1st series) #52]
Minor Thugs Become Big
Additionally, X-Force learned something else while in Camp Hayden: Onslaught was no longer alone. The mysterious being had started recruiting others to do his dirty work for him, in this case the Blob and the Mimic, who had been snatched right under their noses during an earlier conflict with X-Force. Both of them had been given power modifications and upgrades by Onslaught, and in the weeks that followed, Onslaught could convince other villains to become his henchmen. The powerful entity knocked out Sebastian Shaw and Tessa, to lure Holocaust away from them, promising him immeasurable power in exchange for his loyalty. In London, Onslaught approached Ms. Steed, the Black Queen of the Hellfire Club’s London branch, 
making it pretty clear that he wouldn’t accept No for an answer. Getting arrested by the police though spared her from getting involved deeper in Onslaught’s schemes. [X-Force (1st series) #46, 52, X-Man #15, Excalibur (1st series) #99]
The X-Men got to know the last, and possibly most powerful of Onslaught’s lackeys the hard way. Storm, Wolverine, Iceman and Cyclops found themselves teleported away by Gateway, to some battle-field, where they were to be tested by Post, a technologically enhanced mutant and possibly a one man army. Previously, Onslaught had already examined the extent of Chamber’s telepathic power, and now it was the X-Men’s powerhouses. Taken by surprise, the four mutants had to give their all - just to barely survive and eventually overcome Post. However, as Post had mentioned several times during the battle that he possessed only a fraction of Onslaught’s might, and he was right. Right after their safe return home, the X-Men were contacted by Onslaught only to let them know that he was disappointed by their performance, and that there was nothing they could do to stop the Onslaught from coming. [X-Men (2nd series) #49-50]
No Calm Before the Storm
With the threats of Onslaught still fresh in their minds, the X-Men sent Jean Grey and Gambit to infiltrate a secret meeting held at the Pentagon, where Bastion, the leader of Operation Zero Tolerance, met with various world leaders to discuss the threat of mutants, in particular, the being or organization known as Onslaught. Unfortunately, Gambit and Jean’s cover was compromised by Bastion, before they could learn too much. However, while escaping Jean discovered that a mysterious and powerful psychic force had knocked out all the guards in the building. Once outside the building, the ghostly image of Onslaught appeared before Jean and told her to consider this aid as a gift. [Uncanny X-Men #333]
Onslaught’s attention was also occupied by another task; the recruitment of the immensely powerful telepath Nate Grey, a refugee from the world known as the Age of Apocalypse. Onslaught dispatched Holocaust to do the job, as he already had some score to settle with Nate, and he soon found his target in Greece together with his companion, Threnody. Nate found himself on the defensive as he had to protect innocent civilians, but Holocaust still offered the man a chance to join Onslaught, who could save Nate from burning out and dying as he was destined to do. Nate refused, but was forced to surrender when Threnody’s life was put in danger. Using physical force, Nate cracked Holocaust’s armor, though he was unable to breach the powerful psionic defenses Onslaught placed around Holocaust’s mind. Accepting defeat, Holocaust fled, but warned Nate that Onslaught knew his potential now and would come in person to claim him. Unsure what to do next, Threnody suggested for Nate to ask the Avengers for help. [X-Man #16-17]
Meanwhile, Nate’s counterpart in this reality, Cable, enlisted the aid of Domino to once more check out Camp Hayden in hope for some clues to Onslaught’s identity. While empty and abandoned before, Cable and Domino found the facility quite armed with guards this time. Fighting their way through, the pair found the memory chips for the security cameras, which should contain Onslaught’s image, however a closer examination revealed that the chips were magnetized and thus useless. Cable and Domino quickly left, unaware that the whole stint had been observed by Post. Back at the Xavier Institute, Cable was asked by Storm to mindprobe her to learn more about Onslaught’s herald, Post, as she was one of the few people to have actually seen the man. 
While Cable’s efforts were useless, all visual images of Post having been mentally blocked, he couldn’t help but find the psionic imprint to be quite familiar. Before anything further could be done, Cable learned that the base of his mentor Blaquesmith had exploded. Arriving at the scene, Cable discovered that Post was behind the attack. face to face with him, Cable also recognized him as a man he once saved many years ago, Post’s technological enhancements actually being the side-effect of a transfusion of Cable’s techno-organic virus infected blood. During the inevitable fight, Cable realized that there was only one person who could have the knowledge and power to launch so many assaults against the X-Men and correctly guessed Onslaught’s true identity. However, Post beat Cable into unconsciousness before the man could send out a warning to the X-Men. Deeming Cable no longer a threat, Onslaught left to carry out his plans accordingly. [Cable (1st series) #32-33]
The Beast Awakens
Around this time, while Jean Grey was shopping, Onslaught mentally attacked her and took her inside his mind. He then made her witness his awesome might, and taunted her with some of Xavier’s darkest secrets, like that he once had romantic feelings for her, despite being happy for both her and Scott. The being then attempted to show Jean the true nature of human beings, but found that Jean was too resilient and would not be swayed. Onslaught then decided that Jean had seen enough for now and sent her back to the real world, erasing the memory of their meeting, but giving her one warning at the same time: Onslaught is here!
Jean knew that something was wrong but didn’t know what exactly. She went back home, where she found Gambit and Bishop defeated on her porch. The Juggernaut, returned from his other-dimensional exile with the help of Psylocke’s shadow-porting power, found that he could not remember the true identity of the being plaguing them all, was responsible and wanted help in finding out what Onslaught was. Not fully trusting his stepbrother, Charles Xavier, he wanted Jean to telepathically probe his mind instead. Fresh from her own encounter with Onslaught, Jean took the Juggernaut to the psi-shielded Z’Nox chamber beneath the mansion, in order to make sure that Onslaught couldn’t interrupt their quest. After breaking Marko‘s mental blocks, Jean found out who Onslaught truly was and she realized that the Juggernaut was in grave danger for Onslaught was in the mansion. The terrified Juggernaut followed Jean’s advice to run away, but unfortunately ran into his step-brother Xavier instead, who shockingly revealed himself to be the mighty Onslaught and proceeded to rip the Gem of Cyttorak out of his chest, trapping Cain in it in the process. [X-Men (2nd series) #53, Uncanny X-Men #334, X-Men (2nd series) #54]
Jean wanted to warn her fellow X-Men about Onslaught’s identity being Xavier, but right then did he gather the X-Men to make a speech. Jean kept the identity of Onslaught secret from her teammates, knowing that it would be better to learn more rather than to lose the only advantage of surprise. After a long, emotional speech about how the team failed him the X-Men began to realize that something was wrong. Xavier had been acting out of character for quite some time now, like when he told Cannonball to either shape up or ship out, or when he sent a holographic projection of himself to talk with Cyclops, rather than to meet him himself. However, during the speech, Jean began to doubt the notion of Xavier being Onslaught and believed the villain was only trying to trick her, but the sight of the Gem of Cyttorak on Xavier’s desk convinced her that it was the truth. Before Jean could warn the X-Men, Onslaught overpowered her with psionic energy and rendered her useless. As the confused X-Men rushed to Jean’s aid, Xavier finally revealed himself to his students to be the mysterious Onslaught!
The being announced to the horrified X-Men that he was born from Xavier, though dropped hints that this was not the entire story. Onslaught then offered the X-Men to fight by his side and to put an end to the threat humanity posed to mutant kind, but the heroes of course all refused. Onslaught began to display his mighty power upon the team, but was stunned to find that Cyclops would fight back ferociously. With Xavier still struggling against him, the injured Onslaught escaped. A quick look at Xavier’s open files, however, showed the X-Men that Onslaught as of recent had begun to compile information on Nate Grey, as well as Franklin Richards, son of Mr. Fantastic and the Invisible Woman of the Fantastic Four. The team could only guess what Onslaught could want with the two powerful mutants, when Onslaught returned and revealed his plans of a new world order, before knocking them all out. One of the few left standing, Jean rushed into the Z’Nox chamber once more to send out a distress call to the other X-teams. It was a recording of this message that Bishop had seen in the future, and it was his presence that saved the X-Men since Bishop absorbed the brunt of Onslaught’s otherwise lethal blast.
Meanwhile, the Beast finally revealed his true colors, that he was not the real Henry McCoy, but the Dark Beast, his counterpart from the Age of Apocalypse, the same timeline that Nate Grey and Holocaust stem from. He had replaced the Beast a few weeks earlier, and offered to join the ranks of Onslaught’s lackeys. As Xavier began to fight back once again, Onslaught and Dark Beast departed together and destroyed the X-Men’s mansion along the way. With the horrible truth revealed and Onslaught fully awakened, the X-Men lay shattered and it was only a matter of time before the world would feel the might of Onslaught’s mighty hand. [Onslaught: X-Men]