Onslaught was a sentient psionic entity created from the consciousness of two extremely powerful mutants: Professor Charles Xavier and Magneto. During a battle between the X-Men and Magneto's Acolytes, Wolverine slashed Magneto, and in response, Magneto ripped out the Adamantium from his bones, severely injuring Wolverine so much that even his healing factor had trouble healing him. For Professor Xavier, this was the limit. Having tolerated every one of Magneto's evil actions, even his recent actions that had killed hundreds of people across the globe in a single day, he finally snapped. He used his telepathic powers to shut down Magneto's mind, rendering him catatonic. During the psionic contact, Magneto's anger, grief, and lust for vengeance entered Professor Xavier's consciousness; commingling with every long-suppressed negative feeling the professor had endured during the last 30 years. This aggregation resulted in a separate personality within Professor Xavier, the being known as Onslaught.
Onslaught remained dormant for some time. Gradually, Xavier's frustration with his dream of mutant-human peace and harmony began to increase, exacerbated by the daily growth of anti-mutant hysteria, the deaths of dozens of mutants from the lethal Legacy Virus, and his own failed attempt to rehabilitate the mutant sociopath called Sabretooth. Xavier reached his breaking point when anti-mutant humans killed a young mutant near the grounds of the Xavier Institute for Higher Learning, the secret base of the X-Men. It was at this point that Onslaught awakened within Xavier and began to manifest himself to others.
Following an encounter with the mystically-empowered criminal called the Juggernaut (Xavier's stepbrother, Cain Marko), Onslaught invaded a government-sanctioned Sentinel base. He downloaded the current specifications for the mutant-hunting androids and erased portions of the memories of the employees working in the clandestine factory. Soon after, while hiding in Xavier's mind, Onslaught encountered the telepathic, telekinetic Nate Grey. He pulled the Professor's body from the astral plane, a feat never before achieved. Through X-Man's actions, Onslaught was able to escape Xavier's mind and move independently. After taking full possession of the Professor's mind and body, Onslaught-as-Xavier called together the X-Men. When they resisted his plan to turn them into soldiers fighting a total war against humans, Onslaught revealed himself to the X-Men and attempted to destroy the outlaw adventurers. The time-tossed mutant soldier called Bishop, remembering childhood stories of a traitor betraying and killing the X-Men, began to realize that the guilty party was Xavier as Onslaught. Bishop rescued his teammates. Taking Xavier's body with him, Onslaught escaped.
Once free, Onslaught created a childlike psionic projection named Charles to gain the trust of young Franklin Richards, a mutant possessing the latent power to alter reality. Onslaught succeeded in capturing the child despite a rescue attempt by Franklin's parents, Mister Fantastic and Invisible Woman, members of the legendary super-team known as the Fantastic Four. Now carrying Franklin and Xavier, Onslaught captured X-Man, intending to use his raw psionic power to fuel Onslaught’s plans to transform the human race into a collective consciousness. Onslaught returned to New York, reprogramming the Sentinel robots to obey his commands and form a protective circle around Manhattan, sealing off the island from the rest of the world.
The X-Men, the Fantastic Four and the Avengers, Earth's Mightiest Heroes, challenged Onslaught. Their first confrontation with the entity and his minions resulted in the rescue of a now-powerless Xavier, who retained his usual benevolent personality. Onslaught now existed as psionic energy contained within his armor. A second and final confrontation occurred at his Central Park citadel. After a number of attempts to free Franklin and X-Man from Onslaught's seemingly impervious armor, the heroes regrouped. During their next assault, X-Man Phoenix (Jean Grey) used her telepathy to turn off the part of the monstrous Hulk's mind that contained the rationality of his alter ego, Dr. Bruce Banner. The now-savage Hulk, possessed of a near-animal intelligence and unbelievable strength, battled Onslaught and tore apart his armor. The resulting explosion of psionic energy separated Banner and the Hulk, and Onslaught was left as an energy being, immune to most physical harm. Onslaught then used Franklin's powers to create a second sun that would destroy the heroes and Manhattan.
The Avenger known as Thor, Asgardian God of Thunder, master of storm and lightning, flew into the energy being, disrupting Onslaught's form. Realizing that Thor's efforts alone were insufficient to destroy their adversary, the non-mutant members of the Avengers and the Fantastic Four entered the energy field, and the Hulk-less Banner and Dr. Doom (albeit reluctantly) followed. The X-Men were unable to do likewise, as it was theorized that if Onslaught, a being of mutant origin, came to possess a mutant host, he would become unstoppable. Onslaught dissolved, seemingly destroyed, along with the heroes who had thrown themselves into the rift. The remaining Sentinels were disabled, Franklin and X-Man were rescued, and Manhattan was returned to normal. Later, it was revealed that the Avengers and the Fantastic Four did not die, as had been surmised. Rather, the heroes were reborn in a pocket universe created by Franklin. The Avengers and the Fantastic Four returned to their rightful home months later, possessing only hazy memories of their time on Counter-Earth.
Following the Scarlet Witch's proclamation of "No more mutants" and the subsequent depowering of most of the Earth's mutants, a large mass of energy was released into space. After awakening Vulcan and Darwin and being controlled by Xorn (who returned to Earth, fused with Collective and was then defeated by the combined efforts of the New Avengers), the ball of energy returned to Earth a second time and gained physical form as a recombined Onslaught. Immediately seeking out Franklin Richards (and failing to kill him through a mind-controlled Human Torch and Reed Richards), Onslaught quickly cornered the child in his bedroom, where Franklin once more used a toy ball as a focus to escape into a world that was for the most part an exact copy of the Counter-Earth he had created.
There, with the help of recreated versions of the Fantastic Four, Avengers and their allies and foes, Franklin battled Onslaught. The fight took them to the Baxter Building where Bucky sacrificed her life knocking Onslaught into the Negative Zone. Bucky found herself transported to Earth-616 while Onslaught remained trapped in the Zone. He was last seen floating near Wonderland curious of its purpose.
As both Secret Avengers and Young Allies were investigating a Roxxon facility in Colombia, Onslaught emerged from the Negative Zone and possessed Rikki's body. He then reveals that the real Rikki had in fact died in the Negative Zone and she was just a construct made by him so he could use the energy he had stored in her as an anchor to pull himself back into the main Marvel Earth. The revelation that she had been sent to Earth solely to serve as Onslaught's tether, forced her to fight back only to prove he was wrong. Eventually, Rikki had Gravity kill her so that Onslaught could not return and destroy Earth.
Onslaught possessed all of the mutant abilities of his progenitors Professor X and Magneto. He later added the powers of Franklin Richards and Nate Grey to his, making him even more powerful. He was ultimately contained by the heroes only because of the Franklin's intervention to spare their lives, which broke Onslaught's control. According to Franklin, Xavier "told me to fight Onslaught" and that with Nate's help "maybe we can together". 
Astral Plane Tap: Ability to Tap into the enormous psychic resources of the astral plane in order to manipulate matter and energy.
Astral Travel: Capable of forging psi-links to travel in astral form.
Material Astral Projection: Give substance to astral energy to solidify psychic forms or forge protective psi-armor and even pull the astral projections of other telepaths into the physical world.
Psychometry: Can read residual thought imprints left on objects touched by people.
Dimensional Sense: Capable of sensing dimensional rifts or anomalies.
Telepathy: Onslaught is able to read minds and project his own thoughts into the minds of others in vast distances from him with little to no resistance from even the most powerful telepaths.
Telepathic Illusion: Ability to create realistic telepathic illusions and cause people to experience events which are not actually occurring.
Telepathic Camouflage: Ability to mask himself, and other peoples' presence from those around him. Can telepathically disguise himself, making his appearance to those around him quite different (changing the appearance of clothing, as well as more involved disguises).
Mind Blast: Ability to place large amounts of information in another mind.
Mind Control: Ability to control the minds of others upon mere concentration. This power can allow him to completely shut down several peoples' minds, making it appear as if time has stopped. His telepathic control was so powerful that he was one of the only few to completely control Hulk (albeit in a calm state), despite the green goliath's immense resistance to psychic control.
Mind Possession: Ability to possess the mind of another, and use that being's body as his own.
Mind Alteration: Ability to alter the minds of others by force of will.
Mental Amnesia: Ability to cause loss of particular memories, and amnesia in another person or even in a group of people. Powerful enough to make a person have total mental paralysis.
Psionic Shield: Ability to erect a psychic shield for protection of himself and of others minds and is sufficient to protect himself from other psychics on the Omega-level.
Psionic Blasts: Can project psionic force bolts which have no physical effects but which can affect a victim's mind so as to cause the victim pain or unconsciousness and can even kill an adversary. Even without being fully evolved, his psionic blasts affected Juggernaut even though Juggernaut was wearing his helmet which had protected him from very powerful psychics, slowing him down enough for him to hit Juggernaut.
Astral Projection: Ability to Astral Travel and communicate with others astrally through his own will, or through contact with the thoughts and memories of others. In the astral realm, he can use his powers to create "ectoplasmic" objects. He cannot engage in long range astral projection on the earthly plane.
Mental Detection: Can sense the presence of another superhuman mutant within a small but as yet undefined radius of himself by perceiving the distinctive mental radiations emitted by such a being.
Telekinesis: Telekinetically, Onslaught is capable of very fine control over objects; he can disassemble complex devices explosively (separating every last component, such as screws, nuts, circuit boards, etc.), and just as quickly and easily assemble complex devices. This also has enabled him to even attain awareness and control over objects as small as individual electrons in an atom. For instance, Onslaught could detect if a seal was hermetic, or not, by checking for the presence of penetrating oxygen molecules. He could even detect and discern individual oxygen molecules, and determine the atomic weight of molecules, by the amount of atoms present. On larger levels, Onslaught can levitate great weights, into the tons, but his upper limit has remained undisclosed.
Psionic Spikes: Ability to create destructive psionic spikes that destroy the physical objects that the spikes come into contact with.
Force-Fields: Ability to create protective force shields that could deflect even the most powerful of attacks (even filter bacteria from the air). The upper limits of this ability remain unknown, but it is believed that he could probably protect himself from harm at the ground zero detonation of a 1 kiloton nuclear warhead (4.18 terajoules of energy from the heat, concussion, and radiation effects) with ease. Onslaught has also displayed such control over the fields as to shape them precisely, even to conform very closely to his body’s form. There does not appear to be any correlation between field strength and the thickness of the telekinetic fields.
Concussive Blasts: Can project his telekinetic energies as powerful blast beams directed from his brain that could apparently affect matter with concussive force.
Flight: By levitating himself, he can fly at incredible speeds.
Intangibility: Can phase through solid matter by mentally shifting his molecules around another object's as he moves forward.
Telekinetic Holograms: His mental fine motor skills are so acute that he can mentally manipulate dust particles and water vapor to refract light-waves and create holograms.
Electromagnetic Spectrum Manipulation: Onslaught has some ability to project or manipulate any form of energy that is related to the electromagnetic spectrum. He can fire and absorb bolts of electricity and magnetic force, reverse lasers and other forms of radiation or energy, create enough intense heat as infrared radiation to level a city, and become invisible by deflecting visible light around his body. Heat or infrared radiation, is part of the electromagnetic spectrum, which also includes visible light, radio waves, ultraviolet light, gamma rays, and x-rays. Onslaught can project any of these. Onslaught can maintain total control of an entire planet's electromagnetic field and can manipulate it accordingly. He has also dispersed a "flame cage" created by the Human Torch, but whether he had simply expanded his personal force field or employed something else entirely is unclear. Onslaught once used his powers to create a stable wormhole between two points in space.
Electromagnetic Sight: Onslaught can perceive the world around himself as patterns of magnetic and electrical energy. He can perceive the natural magnetic auras surrounding living beings.
Electromagnetic Pulses: He also has creative uses of his powers such as using his telekinesis to channel the Earth's magnetic field through the electro-current of his brain and generate devastating electromagnetic pulses.
Magnetokinesis: Onslaught has the power to shape and manipulate magnetic fields that exist naturally or artificially. He can draw magnetic force from outside himself, or can also generate magnetic force within himself. Onslaught's power is psionic and physiological in nature. He can lift, move, and alter objects. Onslaught can project metal at an unparalleled velocity and place tremendous pressure on metallic substances to liquefy and reshape them, manipulate the iron-enriched blood-flow to one's brain to induce aneurysms or unconsciousness, redirect the flow of blood through one's brain to alter thoughts and perceptions or blank a person's mind completely, levitate a person or control a person's actions by manipulating the iron in their bloodstream, remove the iron from the bloodstream entirely through a person's skin, control ferrous particles in the atmosphere, alter the Earth's magnetic field which extends into space as the Onslaught sphere, prodigiously increase his own strength to superhuman levels, and achieve a wide range of other effects.
Magnetic Force-Fields: Onslaught usually protects himself with a personal force field that he can quickly expand to protect large areas. His force field has withstood the effects of multiple nuclear weapons, volcanic eruption, the depths of space and attacks from multiple Avengers or X-Men, including Phoenix, Thor, and even Galactus.
Organic Iron Manipulation: Onslaught can control the iron within any organic matter which allows him to have full control over an opponent's body. Onslaught has also demonstrated the ability to control the blood flow within one's body thus causing them to die a painful or even quick death.
Superhuman Strength: Onslaught possesses vast superhuman strength of unknown limits. Onslaught can lift well in excess of 100 tons easily without using his psionic, magnetic, telekinetic or reality altering powers to increase his strength. While using those powers to enhance his strength, the upper limits of Onslaught's strength are unimaginable. While not fully evolved physically, he demonstrated sufficient strength to overpower a fully powered Juggernaut and punched him with enough force to render him unconscious and send him flying from Canada to New Jersey with just a single blow. He later used his claws to easily skewer the Juggernaut and rip out his Cyttorak gem from his body.
Impervious Armor: After his last physical evolution, Onslaught’s armor is nigh-invulnerable and nearly impossible to crack. The only exceptions were blows from an enraged Banner-less Hulk. However, some were capable to cause damage when Onslaught was in an unstable vulnerable mid-evolution flux. Finally, Onslaught used the Hulk's phenomenal strength to destroy his armor so that he could fully evolve. Hence it presumably means that he could not evolve by himself and the Hulk needed to overcome his powers.
Telepathic Immunity: His mind is immune to all traces of telepathy, due to his armor, his mind cannot be manipulated nor can it be read. Although, his telepathy is capable of exiting his armor, allowing his telepathy to be used on others.
Mutant Absorption: As a psychic entity, Onslaught had the ability to absorb any mutant into his being and thus permanently gain their powers for himself. X-Man and Franklin Richards were two mutants that he consumed to further his vast power.
Reality Manipulation: Ability to manipulate reality itself. Reality warping powers on a potentially cosmic scale. His abilities are so powerful that he can create "pocket" universes in alternate realities (A pocket universe is a cosmos that does not equal our own in size). Utilizing this power he was able to effectively create a second sun from nothing, something only high level cosmic entities have been able to perform.
Genius-level Intellect: Onslaught possesses the combined genius intellects of both Charles Xavier and Magneto, granting him the same expertise and knowledge of each. With his super-genius intellect he could master his powers better than inexperienced wielders, like the reality manipulator Freedom Ring or the infant and child Franklin Richards.
Formidable hand-to-hand combatant: Onslaught is also possessing the knowledge of combat both Xavier and Magneto have learned over the years.
Strategist: Onslaught is particularly adept at using all of his powers in combat situations, typically preferring to combine certain aspects of different abilities for different situations. He is a skilled manipulator with and without his powers. He has tricked his enemies into doing exactly what he wished to do multiple times.
There is a continuity error in Onslaught's final defeat. It was theorized that if a mutant were to be absorbed by Onslaught's energy form, Onslaught would in turn become invincible. However, of the members of the Avengers who selflessly sacrificed themselves to defeat Onslaught, two were mutants:
Scarlet Witch, although it has been suggested that her Hex powers somehow shielded her
Earliest possible genesis for Onslaught may hark as far back as X-Men #25. In the particular issue Professor X shuts down Magneto's mind following Magneto's extraction of Wolverine's Adamantium skeleton.