When Charles Xavier telepathically rendered Magneto catatonic, he unknowingly absorbed Magneto’s dark ego. Magneto’s negative emotions merged with Xavier’s suppressed urges, forming a powerful, dormant psionic entity - the self-dubbed Onslaught. Rising anti-mutant hysteria, Legacy Virus casualties and Xavier’s failure to rehabilitate Sabretooth all increased Xavier’s frustration. When anti-mutant humans killed young mutant Dennis Hogan near the Xavier Institute, Onslaught finally awakened within Xavier, and it decided to achieve Xavier’s dream of peaceful mutant/human coexistence by utilizing the interdimensional firm Landau, Luckman & Lake’s warp portals to bond humanity into a collective intelligence. To this end, Onslaught attempted to generate fear to heighten psionic activity. In Vancouver, B.C., Onslaught sought to recruit Xavier’s step-brother Cain Marko, the criminal Juggernaut. When Marko declined, Onslaught mentally blocked Juggernaut from remembering its true identity, and magnetically threw him to New Jersey.
Learning the mutant-hunting Sentinels could store psionic energy, Onslaught abducted scientists to create a Sentinel army. Soon after, Xavier encountered X-Man (Nate Grey) on the astral plane, who pulled Xavier’s astral form into the physical world. Onslaught thus escaped Xavier’s mind, but was still “tethered” to Xavier. To assist disseminating his hyper-psionic energy, Onslaught had the mutant teleporter Gateway abduct the psionic mutant Chamber, sent the armored mutant Post to “test” the X-Men and Cable, and recruited and empowered other mutants such as Blob, Mimic and Holocaust. Now fully possessing Xavier, Onslaught revealed Xavier’s suppressed darker memories to Jean Grey, hoping to recruit her Phoenix Force experience to gain more power. Rejecting Onslaught, Jean removed the Juggernaut’s mental block, discovering that Onslaught was Xavier. Onslaught then trapped Juggernaut’s consciousness inside Marko’s own empowering Ruby of Cyttorak, and created the child-like psionic projection Charlie to befriend and capture vastly powerful mutant child Franklin Richards.
As Xavier, Onslaught attempted to lead the X-Men to war against humans. When the X-Men resisted, Onslaught revealed itself and attempted to destroy them. Onslaught then mentally enslaved the Hulk and sent him after Cable, but Cable and Storm freed the Hulk. Onslaught captured X-Man, adding both his and Franklin’s psionic powers to its own. It sent the Sentinels to Manhattan to sow panic, created a Central Park citadel, and unleashed an electromagnetic pulse. The X-Men, Fantastic Four, Avengers and even Doctor Doom opposed Onslaught, and after Thor freed Xavier, Onslaught became even more powerful.
Concluding that no one was worthy to live, Onslaught decided to obliterate humanity and created a second sun. While Thor and Storm sought to minimize the resultant climactic damage, the Hulk had Jean Grey turn off his human mind, resulting in a rage-fuelled Hulk who shredded Onslaught’s psi-armor. Onslaught exploded, separating the Hulk from his human side, and became a being of pure mental energy. Thor began disrupting Onslaught’s form from within, and the other Avengers, the Fantastic Four, and several non-mutant allies weakened it further. As Onslaught dissipated, both Franklin and X-Man were rescued. Though believed dead, the heroes were actually transported to a pocket universe subconsciously created by Franklin where they briefly encountered a manifestation of Onslaught, heralding his future return.