In Uncanny X-Men #12, Professor Xavier fills the X-Men in on the Juggernaut's origin. Charles Xavier was a smart and sensitive young boy. He had an older stepbrother named Cain Marko, however, who was a boor and a bully. They were rivals their whole lives, although Xavier was able to get the upper hand once his psychic powers manifested. As young adults, the two were overseas (originally because they were fighting in the Korean War, although that got left out of later versions) and happened upon a cave that contained "the sacred, lost Temple of Cyttorak." Cain grabbed a ruby from a strange-looking statue, and it transformed him into the unstoppable behemoth known as the Juggernaut.
The cave collapsed with Xavier barely escaping, and years later, Marko hunted down his hated stepbrother to destroy him. The Juggernaut's distinctive helmet makes him immune to Professor X's psychic powers, and that first battle with the X-Men, like many that followed, revolved around the X-Men maneuvering the Juggernaut into a position where they could get his helmet off, enabling Xavier to lay down the psychic whammy.
