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 has trouble healing him. For Professor Xavier, this was the limit. Having tolerated every one of Magneto's evil actions, 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.