And in different Universes, there were different reasons for the birth of Onslaught.
In Universe 235, it happened during a battle between the X-Men and Magneto's Acolytes.
Magneto extracted the Adamantium from the skeleton of Wolverine. Upon seeing this, a very angry Xavier used his telepathic powers to shut down Magneto's mind, rendering him catatonic.
Although he saved them, when he returned to his room, Magneto's anger, grief and lust for vengeance entered Xavier's consciousness; that and with every long-suppressed negative feeling Xavier had endured during the last 30 years.
This aggregation resulted in the being born was known as Onslaught, a being made of psionic energy combining Magneto's and Xavier's psyches.
Onslaught came to the X-Men team and he claimed he was Charles Xavier, but since his appearance was similar to Magneto but was larger.
Wolverine remarked that he looked more like Magneto, with the helmet, build, and color scheme.
These statements made Onslaught very angry, and he then released a huge amount of energy, destroying the room they were all in.
He proceeded to kidnap Jean Grey and took her to the astral plane.
He tried to convince her to join him and his cause but without revealing that he was part of Xavier's subconscious.
Onslaught then tries to show Jean Xavier's repressed frustration caused by his love for Jean.
As a result he attacks Jean and burns his name psionically on Jean's forehead, which only Jean can see. Then he throws Jean out of the astral plane.
Jean and the X-Men later meet up with Juggernaut who asks Jean to search through his mind telepathically to find out who Onslaught was.
Jean placed her hands on Juggernaut's head, and proceeded to look for information, which led to her discovering that Onslaught knew where they were.
However, Juggernaut ran, and once in the hallway, found himself in a maze of stairs, and found only a door in which to escape.
He ran through the door, only to find a massive Onslaught on the other side. Onslaught tore the Gem of Cyttorak from Juggernaut's impenetrable body, and he would soon be kidnapped by Onslaught.
Later, back in Xavier's body, Onslaught called the X-Men into his study. He claimed that he wanted to destroy the human race to leave the mutants supreme.
Jean, distressed at what Xavier was saying, walked around the room, and saw the Gem of Cyttorak on the table, while Juggernaut inside.
Onslaught then proceeded in battling the X-Men and destroyed most of the school. He continued his plans to try and destroy all humanity.
Meanwhile, Wolverine and Elektra were able to figure out that Onslaught was in fact Xavier and Magneto.
Onslaught intended to destroy all mutants and humans by absorbing the powers of Franklin Richards and Nate Grey to increase his own psionic powers.
Onslaught battles the Avengers, Fantastic Four, The Hulk, the X-Men and Doctor Doom.
During the battle the Hulk destroys Onslaught's psychic armor with a powerful punch, so powerful that the Hulk and Bruce Banner separate.
Before Onslaught could do more harm, Ben entered the scene and defeated him.
All except Franklin Richards, Nate Grey, Quicksilver, the Banner-less Hulk, the Magneto clone known as Joseph and the X-Men are presumed dead.
In reality Franklin created a pocket dimension where he sent the heroes.
And just like this one similar event happened in Universe 367.
After using her powers to de-power Earth's mutants, Scarlet Witch unwittingly causes the powers of Charles Xavier and Magneto to collide and gives birth to Onslaught.
This Onslaught became larger, and gained a Magneto-like mask. He soon faced-off against the Avengers, where he gained the power of possession.
With it Onslaught possesses the Hulk and uses him to attack Thor to see which hero was more powerful.
Onslaught switched between the two heroes, and ended up possessing Thor, striking Hulk with lightning.
As the Onslaught possessed Thor was about to strike Hulk again, he dropped Mjolnir, no longer being worthy. Thus Onslaught switched back to Hulk, and punched Thor high into the atmosphere.
He then went to pick up the hammer, the Avengers arrived and a fight ensued. Captain America then ordered Human Torch and Iron Man to rescue Thor, telling them they had about sixty seconds before Thor reverted to his alter-ego.
They managed to save him in the allotted time, and Johnny took Thor back to the Baxter Building.
Captain orders Hawkeye to go and search for Onslaught, while underneath Hawkeye's costume is Wolverine.
They continue the battle with the possessed Hulk, where Captain is thrown into a building.
Iron Man charges down on Hulk at full speed. He punches Hulk, knocking him unconscious.
He and Captain then assess their next move, when Onslaught possesses Iron Man, and attacks Captain.
But seeing the situation was getting out of control, Ben helps them in defeating Onslaught and sends him to Area 42 prison in the Negative Zone.
Back to the present situation, the Onslaught was standing in front of the X-Men, S.H.I.E.L.D, Spider-Man, Fantastic Four and Elite Mutant Force.
This Universe Onslaught was born because Charles Xavier and Magneto wanted to defeat Mr. Sinister, but they didn't know the power, they couldn't control it, and a new will was entered in their mind.
"What in the web-slinging world is that?" Spider Man asked while his eyes opened wide.
Reed Richards replied while analyzing the situation, "Extraordinary. I've never encountered a psychic force of this magnitude."
"Get ready, people. We're dealing with something we've never seen before." Nick Fury commands his agent.
Captain America said in serious mode, "Avengers, be on guard."
Iron Man tells while scanning "I can't find any data on this entity. Jarvis, what are we dealing with?"
Jean Grey tried to communicate telepathically, "There's a presence, a powerful one. I can't... It's like a storm in my mind."
Scott Summers, while taking care of his brother Vulcan, shouted, "What the heck is that?"
Ben then explains to them how the Onslaught was born, while he activated the power of Chill Dreamer.
The fusion of Big Chill and Pesky Dust, since he has experienced defeating Onslaught in other different Universes.
Onslaught, attempting to exploit his telepathic powers to delve into Ben's mind. However, he isn't able to enter his mind.
Ben taunted him, "You can't read my mind, Onslaught. Nice try."
"We will see," Onslaught responded, determination resonating in his voice.
He shifted his tactics, attempting to enforce his will through mind control. Yet, like his telepathic abilities, this too proved futile against Ben's resilient mental defenses.
Seizing the opportunity, Onslaught unleashed a barrage of powerful energy blasts, aiming to overwhelm Ben.
However, the icy fusion seemed untouched, the energy dissipating harmlessly against Ben's invulnerable form.
Ben, now confidently standing amidst the dissipating energy, quipped, "Is that all you've got?"
His eyes glowed with a mix of ice and stardust, ready to counter whatever Onslaught would throw next in this cosmic clash of powers.
"Your mind is resilient, but I will break it." Onslaught shouted while gritting his psychic teeth.
In a swift move, Onslaught teleported behind Ben, attempting a sneak attack.
However, Ben's effortlessly evade the surprise assault.
"I already said, you can't do anything to me," Ben again mocked him.
Onslaught continued his attack,while Ben relied on his quick reflexes, evading Onslaught every attack.
"You can't escape forever, Ben." Onslaught said. He decided to tip the battle in his favor.
Using a magnetic surge, he manipulated the environment, hurling cars, buses, and even parts of buildings toward Ben.
"Let's see how you dodge this!" The debris rained down, a chaotic storm aimed at him.
However, Ben, utilizing his intangibility, phased through every attack emerging unscathed.
"Try more, but I've got more than one trick." Ben replied while grinning.
Onslaught, growing more frustrated with each thwarted attempt.
With a menacing grin, he unleashed the full force of his magnetic manipulation, this time not only the car and buses, but airplane, train and even the full building itself moves to attack Ben.
And if he phases it again, it will attack on the rest of the heroes there.
"Let's see how your tricks save them!" Onslaught smirked.
In response, Ben using Absolute Cryomancy, it can freeze any thing, and more fast.
With precision, he froze the incoming projectiles before shattering them into harmless pieces, protecting the others.
"Nice try, but it is again a futile attempt."
Onslaught took a darker turn, seizing control of Thor's mind, The God of Thunder.
He launched a powerful assault against Ben. Yet, Ben effortlessly countered Thor's attacks.
Undeterred, Onslaught switched his focus to the Hulk, manipulating the green behemoth to charge at Ben.
But Ben managed to hurl Hulk back with surprising ease.
Onslaught, however, wasn't finished. He turned his attention to Tony Stark, compelling Iron Man to fire a barrage of attacks.
Simultaneously, Captain America, under Onslaught's control, charged with his shield, while Hawkeye launched a volley of arrows.
Ben, with his invulnerable body, endured the onslaught. For the attacks that managed to find their mark, Ben phased through them, emerging unscathed.
Then Onslaught's psychic grip tightened on the minds of the X-Men.
Jean Grey, Storm, Colossus, Angel, Dazzler, Nightcrawler, and Wolverine moved with coordinated precision, their expressions blank as they advanced on Ben.
"Unleash your power upon him, X-Men!" Onslaught cheered while manipulating X-Men
Ben soared through the air, when Storm unleashed bolts of lightning. And he evaded effortlessly, each attack.
"Is that all you've got?" Ben mocked him.
Colossus, fueled by Onslaught's command, delivered a thunderous punch, but Ben, invoking his Titanic Endurance, stood unfazed.
"Your resilience won't save you, Ben. X-Men, don't hold back!" Onslaught said, while controlling X-Men.