Cain Marko is the son of Kurt Marko, who becomes Charles Xavier's stepfather when he marries Sharon Xavier after the death of her husband Brian, for which Kurt is partially responsible. Kurt Marko favors Charles and abuses his own son, Cain. Cain resents Charles and bullies him frequently. Cain Marko and his step-brother Charles serve in the US Army and are stationed in Korea.
When Cain Marko finds the Gem of the mystical entity Cyttorak, he is empowered with magical energies and transformed into an immortal avatar for the entity in question. As the Juggernaut, Marko possesses superhuman strength, being capable of shattering mountains, lifting and using buildings as weapons, and extreme durability.