Cain Marko was the spoiled child of a famous scientist, partner to Brian Xavier, who, after leaving him die in an accident, married his widow and so, Cain became foster brother to Charles Xavier. Due to his attitude, the relationship between the kids was never good, but it became even worse after Charles unwillingly invaded Cain's mind when his telepathic powers appeared in his youth.

Later, during the war, Xavier tried to stop his foster brother from deserting, following him into a temple where the man found a red gem, the ruby of Cytorak, which transformed him into a giant juggernaut. Xavier, realizing how dangerous his brother had turned, escaped from the wrecking temple, where Cain got buried, but even then he knew that the Juggernaut would return for him ...

Some years later, after Xavier had founded the X Men, the Juggernaut came for him. Until then, the weak point of the man had been his mind, vulnerable to a psychic attack, but he had found a helmet to protect himself and was unstoppable. Fortunately, the combined effort of the first X-team defeated him, who allied to Black Tom Cassidy (Banshee's cousin) to take benefits from his strength as criminals. Together they brought up Syrin, hiding her existence to his father.

His path has crossed many times with the X Men, like when he knocked out Colossus for spilling his beer, while he was flirting with Selene (ulp!) but lately, he has proven to be no rival for the mysterious Onslaugh, coming to the X Men to ask for help and exposing his mind to Jean Grey's powers to show his good will. Will they find out who is Onslaught?