The psionic being known as Onslaught was first created when an enraged Charles Xavier attempted to shut down the mind of his old enemy Magneto after the master of magnetism ripped the adamantium off Wolverine's body during the "Fatal Attractions" storyline, planting the seeds of Onslaught's manifestation.
That fateful decision would result in the birth of one of the Marvel Universes' most dangerous and powerful monsters, and despite his apparent death after each of his dark rebirths, Onslaught keeps finding new ways to return and torment the heroes and villains of the Marvel Universe, particularly young heroes like Franklin Richards and Rikki Barnes/Nomad.
10 TRIED TO STEAL MUTANT POWERS
While Onslaught had access to the powerful telepathy of Professor Xavier and the elemental magnetism of Magneto, he sought out the abilities of other young mutants like Nate Grey and Franklin Richards to add to his power.
He created a youthful psychic avatar named Charlie to try and coerce young Franklin into joining him before he decided to kidnap him outright. Onslaught then used Franklin to create a lure that allowed him to trap Nate Grey as well, adding both of their immense power to Onslaught's and granting him the ability to alter reality.
9 INVADED MANHATTAN WITH SENTINELS
Onslaught may have had access to untold levels of power after he absorbed the abilities of four different omega-level mutants, but it took him a bit of time to gain control of the abilities as he continued to evolve, so he first stole an army of Sentinels.
Onslaught created a massive Citadel in the middle of Central Park with his new reality-shaping abilities and then closed off the entire island of Manhattan with his fleet of Sentinels that caused a massive amount of destruction to the city.
8 CREATED THE STARK SENTINELS
There have been a few different versions of Onslaught over the years, with one of the most terrifying arriving during the AXIS event, which saw Red Skull (who had grafted Xavier's brain onto his own) transform into Red Onslaught.
Planning for this transformation, Skull had used his new telepathic abilities to influence Tony Stark into creating the Stark Sentinels, powerful weapons specifically built to take out Marvel's heroes that he unleashed as Red Onslaught. Thankfully Magneto had assembled a team of inverted villains that defeated the Stark Sentinels.
7 TRIED TO KILL FRANKLIN RICHARDS
After Scarlet Witch decimated the mutant population after the House of M event, Onslaught managed to use the energies of the event to reconstitute a body for himself, and he quickly sought out Franklin Richards to kill him and regain his power.
However, Onslaught's own abilities were limited at this time, and he was forced to possess the bodies of Reed Richards and the other Fantastic Four, turning Franklin's family against him which forced him to run away to the assumed safety of his own created Heroes Reborn universe.
6 TOOK OUT THE HEROES REBORN AVENGERS
After the lost heroes had returned to the original universe, Franklin recreated his Heroes Reborn universe with new versions of that reality's Avengers and Fantastic Four, which he used to protect him from the threat of Onslaught.
Unfortunately, Onslaught was able to follow Franklin to his pocket universe, where he began to possess and take out powerful members like Thor and Hulk in his attempts to kill Franklin, though he was stopped by Rikki Barnes/Bucky, who trapped them both in the Negative Zone.
5 KILLED MARVEL'S GREATEST HEROES
The climax of the first Onslaught event resulted in the tragic loss of the majority of Marvel's greatest heroes as they sacrificed themselves en masse to give the now completely psionic Onslaught a physical form.
Captain America, Iron Man, the Avengers, the Fantastic Four, Hulk, and Doctor Doom were able to contain the psionic energies so that the X-Men, who had previously almost been wiped out by their former mentor, were able to finally destroy Onslaught. The heroes only survived due to Franklin Richards' creation of the Heroes Reborn universe.
4 ENSLAVED COLOMBIAN CHILDREN
Onslaught's latest attempt to return came during the Onslaught Unleashed storyline which paired the Secret Avengers and the Young Allies together in a Colombian facility that had previously been used to experiment on children.
Onslaught enslaved the surviving community of children and telepathically controlled them as zombified weapons against the combined teams, though the children were eventually released and left in the care of Young Allies member El Toro, who had grown up in the same facility.
3 KILLED AND RESURRECTED RIKKI BARNES
Rikki Barnes was originally created by Franklin Richards in the Heroes Reborn universe as a new version of Bucky, though after her sacrifice to stop Onslaught she found herself in Franklin's home reality, where she became the new Nomad.
Onslaught's most recent return in Colombia revealed that he had actually consumed Rikki while they were lost in the Negative Zone, and then recreated her to serve as his tether, using her as a pawn to return once again. Rikki sacrificed herself again to stop Onslaught but recently reappeared again, seemingly fated to be infinitely reborn.
2 TRIED TO CREATE A COLLECTIVE CONSCIOUSNESS
While Onslaught's follow-up appearances were largely focused on either the murder of Franklin to gain his abilities or the complete destruction of the human and mutants who had stopped him before, he initially followed a different dark dream.
As Onslaught was born of Xavier and Magneto's desires, he was torn between the subjugation of humanity by mutantkind and the peaceful unity of the two races, so he decided to forcibly create a collective psionic consciousness of everyone on the planet that he would control, darkly twisting the ideals that created him.
1 WORLD WAR HATE
When Magneto attempted to kill the Xavier-powered Red Skull during the AXIS event, he unknowingly triggered Red Skull's transformation into the powerful Red Onslaught which increased his powers exponentially.
This allowed him to extend his telepathic reach across the whole globe and spread his genocidal hate across the entire planet and launching "World War Hate" as the planet erupted in madness and violence, though he was eventually stopped and first inverted to the White Skull (controlled by Xavier) before returning to his Red Skull form.