The Birth: Onslaught raised his ugly head for the first time in X-Men (vol 2) #54 after being created by Mark Waid, Scott Lobdell and Bob Harras.
Onslaught is an amalgamation of Charles Xavier and Erik Magnus Lensherr, better known as Magneto.

The Legend: A massacre in New York, which has been made obviously by mutants almost leads to a war between humans and mutants. The name ONSLAUGHT is heard the first time. Hank McCoy, Psylocke and Bishop heard it the first time, when they found a broken Juggernaut in New Jersey. He told them that he was thrown by a being named ONSLAUGHT from Canada to New Jersey. Never before they had seen Cain Marko like this.
.... Senator Kelly finds out about a farm in the middle-west, where Sentinels have been stationed. As he and his troops enter the farm, the lights go on and on all screens there is written one single word: ONSLAUGHT! Someone seemed to have taken control of the Sentinels.
.... The villain had already attacked Cain Marko - who used to pick on Xavier all the time when he was young. Now it went after Jean Grey - a student who Xavier fell in love with but could never haver her. Using her powers Jean learns what Onslaught really is and what his intentions are. The truth shatters all her believings, it changes her live forever and she knows, that the X-Men alone cannot defeat this enemy.
.... In a fight against Magneto, Xavier used his mental powers to shut down Magneto's mind. In the process Xavier's mind was inadvertently infected by the evil within Magneto's psyche. The result was that the dark sides of Magneto's and Xavier's minds combined and physically manifested themselves as the nearly invincible entity called Onslaught, who usurped Xavier's psionic powers. Onslaught battled all of America's heroes, in the process transporting them to an alternate universe; leaving only X-Men to fight the threat. Before the X-Men destroyed Onslaught the demon took away Xavier's powers. After the incident, Xavier surrendered himself to the authorities.

X-Men #54