From Cassius Comics

See Professor X.

When Magneto ripped the adamantium off of Wolverine’s skeleton and pulled it out through the pores in his skin, Charles Xavier was pushed over the line and used his telepathic abilities to erase the dark portion of Magneto’s mind. Unbeknownst to Xavier, the darkness was transferred into his psyche, were it lay festering for months. When Onslaught first manifested, he attacked the Juggernaut in the Canadian wilderness and struck him so hard that the Juggernaut landed in downtown Manhattan. Onslaught later dragged Jean Grey onto the psychic plane and mentally thrashed her. He revealed that as Xavier, he had always harbored a secret attraction to Grey, and Onslaught left Grey unconscious in a clothing store’s dressing room with his name burnt into her mind. Meanwhile, the Juggernaut had had his knowledge of Onslaught’s true identity mentally blocked, and he traveled to the Xavier Institute to ask for Grey’s assistance. They traveled to a psi-shielded chamber beneath New York, where Grey determined that Onslaught was in fact Xavier, but it was too late. Onslaught attacked the Juggernaut and ripped the Ruby of Cytorrak out of his very body; Onslaught trapped Juggernaut within the jewel and added its immense power to his own. Now fully prepared, Xavier revealed his identity to the X-Men and transformed fully into Onslaught. He left the X-Men for dead, except for the Dark Beast, whose identity he had been secretly disguising since he infiltrated the X-Men. Onslaught took on the Dark Beast as a servant, and set out to steal the power necessary to remake the world in his image. By this time, Onslaught had developed the same superhuman abilities that Magneto possessed, and he added to this by capturing X-Man and adding his power to his own. Onslaught had also been communicating with Franklin Richards for some time, posing as “Charley”, a young version of himself. Both Mr. Fantastic and the Invisible Woman wrote Charley off as their son’s imaginary friend, which allowed Onslaught to capture Franklin and channel his reality-altering power as well. Interspersed with a number of confrontations with nearly every New York-based team of heroes, Onslaught’s master plans slowly came to fruition. He took control of an army of Sentinels, and constructed a giant citadel in the middle of Central Park. With everything at stake, the heroes of New York mobilized. The Sentinels were fought back by S.H.I.E.L.D., the Punisher, Spider-Man (Ben Reilley) and the Green Goblin (Phil Urich) while the Avengers, the Fantastic Four, the X-Men, the Hulk and Dr. Doom assaulted the citadel directly. In response, Onslaught used Franklin’s powers to create a second sun above the Earth, threatening life itself. After a failed plan by Dr. Doom, the heroes broke through Onslaught’s defenses and engaged him in direct combat. With his outer shell damaged, and with Xavier’s good side struggling against Onslaught from within, the heroes were able to rescue X-Man and Franklin. The Hulk then attacked Onslaught in a battle so monumental that reality itself was altered by their blows. The Hulk was split apart into two separate bodies, his own and that of Bruce Banner, and Onslaught’s physical form was completely decimated. This left him as a being of pure energy, and his victory seemed assured until The Avengers and the Fantastic Four dove into the middle of his form, seeking to trap his essence in their own bodies. Due to Onslaught’s mutant nature, the X-Men and Quicksilver were unable to join in the attempt for fear of returning his power. The Scarlet Witch and Namor were able to help out due to her hex power and his half-breed nature, and the Falcon (an unconfirmed mutant) was able to help, but it is unclear why. Onslaught was finally defeated when the powerless Banner threw himself into the fray, providing the last necessary body to disperse Onslaught completely. The heroes were seemingly killed, but they were in fact rescued by Franklin Richards, who unknowingly transported them to Counter-Earth, a world of his own creation that rotates the sun directly opposite the Earth. After Onslaught was destroyed, Xavier transformed back to his original physical form.

Upon transforming into Onslaught, Xavier developed a red and purple armored form vaguely similar to Magneto’s costume but with skeletal features underneath.