A deafening screech coming from Cerebro sends the X-Men running to Xavier. Fearful of the impending danger, Xavier instructs the X-Men to set traps on the perimeter of the Institute.
After the traps have been set, Xavier reveals to the X-Men that their soon-to-be attacker is his stepbrother, Cain Marko. Xavier’s father was killed in a blast, and his fellow scientist and Cain’s father, Kurt Marko, tricked Xavier’s mother into marrying him so he could have access to the wealth of the Xavier family. Cain soon came to live with them. Xavier’s mother, heartbroken that Kurt had used her, died shortly after. One day, Xavier overhears Kurt and Cain having an argument and Xavier learns that his father’s death was not an accident. A short-tempered Cain knocks over some test tubes and causes an explosion in the laboratory. Kurt sacrificed his life to save both the boys, and with his dying breath, cautions Xavier to keep his “power” (which he wasn’t aware of) a secret.
Suddenly, they see an approaching figure making his way through all the traps. A snapped electric steel cable manages to stun him for awhile for Xavier to finish his story.
As Xavier grew up, he became more and more aware of his telepathic abilities. He excelled in academics as well as sports, leaving Cain extremely jealous. One day, Xavier had enough of Cain’s constant bullying and retaliated and used his telepathy to his advantage in the fight. To get back at Xavier, Cain tricked Xavier into the car with him and drove madly until Xavier surrendered in fear. That ended in an accident, and Xavier was hurt trying to save Cain’s life.
Cain is now past the steel cable, the grenades as well as sleep gas, leaving Beast puzzled as to how a normal human man has superhuman abilities.
Xavier continues his story, explaining that Cain got his powers through the Cyttorak crystal. It was during the Korean war that both brothers chanced upon the lost temple of Cyttorak. Upon touching the crystal, Cain triggers a cave-in. Xavier escapes and Cain is trapped in there for a few good years.
Cain now goes by Juggernaut and is out to destroy Xavier, having tracked him down. As the Juggernaut breeches the last of the traps, the X-Men spring into action to secure the steel wall sealing the entrance. But even that isn’t enough to stop the Juggernaut and he goes straight for Xavier.
(X-Men: The Early Years #12 is a reprint of Uncanny X-Men (Vol 1) #12.)