Juggernaut is one of the most iconic X-Men villains, and his history of crashing through the Marvel Universe makes him an easy-to-understand fan-favorite character. Between other Marvel powerhouses like the Hulk, Colossus and the Thing, he's fought some of the most physically imposing heroes in the Marvel Universe. He's afforded such power thanks to the Crimson Gem of Cyttorak, an artifact that channels the chaotic energy of a deity known as Cyttorak. Since he was introduced in Stan Lee, Jack Kirby and Alex Toth' X-Men #12, Marko ran amok, either alone or part of a team like the Brotherhood of Evil Mutants or with his common running partner, Black Tom Cassidy.
However, Juggernaut was once considered an X-Man, and now, we're taking a closer look at this period of Cain's life and it all fell apart.
In 2003, Juggernaut started hanging around the X-Men in Chuck Austen and Kia Asamiya's Uncanny X-Men #416 before officially joining the team in Austen and Phillip Tan's Uncanny X-Men #425. Juggernaut found himself in rare peril when his deity, Cyttorak was displeased with him, taking a portion of his power. Greatly diminished, Marko was left at a loose end with little to no direction. In a surprising turn of events, he sought reconciliation with his estranged brother, Charles Xavier. Despite being an antagonist to his brother's X-Men for so long, the X-Mansion welcomed him, with Cain settling in quite well. While he wasn't a mutant, he would find himself numbered among the X-Men soon enough, because although he wasn't as powerful as usual, he was still quite useful in the field.
Among his new friends, Juggernaut made a particularly fond connection with Samuel Paré, also known as Squidboy. The young mutant helped Marko better understand the plight of mutants, as well as other positive influences. Among his new friends, he would come to consider Squidboy the most important. Cain's hot-headedness would see a noticeable reduction during his time with the X-Men. At one point, he even allowed himself to be taken into custody by the law for outstanding charges against him from his previous life of crime and villainy. This showed a maturity and responsibility in Marko that wasn't present before, and he was exonerated of his crimes and allowed to return to the X-Men.
The Brotherhood of Evil Mutants reappeared in a new incarnation, one that included Cain's old crime partner and longtime friend, Black Tom Cassidy. Juggernaut, joined up with his former team in its new form/ However this time he was simply working as a sleeper agent. The plan was for Cain to infiltrate the team, but things took a tragic turn in Austen and Salvador Larroca's X-Men #162. Marko had not informed Squidboy of this plan, and the young mutant reacted harshly when he happened upon the Brotherhood laying out their plans.
Condemning his new friend and admonishing the villains, Squidboy was swiftly killed by Black Tom Cassidy. This sent Juggernaut into a violent rage, in which he tried to kill his former best friend to avenge his new best friend.
Black Tom's murdering of Squidboy proved painfully symbolic for Cain Marko, as it perfectly showed how no matter how he tried, his former life would always come back to haunt him. Having his past take away his future seeded a great guilt within the unstoppable one, with him wishing he could start over clean with everything. Tragically, this wasn't possible, and Juggernaut has to live with the painful memory of Squidboy's last words to him being that he hated Cain. In an attempt to yet again try his hand at a new direction, Juggernaut would later travel to London to join with Excalibur.
Juggernaut would return to the X-Men proper when a gathering assembled to protect Xavier during World War Hulk. While he had regained some measure of his power from Cyttorak, he was still found wanting when Banner came calling. He appealed to Cyttorak once more for his full complement of power, to which the deity mocked him for his weakness in trying to turn over a new leaf. The chaotic being informed Cain that he could only enjoy the full power of the Crimson Gem if he gave in to his most selfish aspects. Marko agreed, gaining the power to bring the fight to the Hulk. Unfortunately for him, rather than try and fight him, Hulk outsmarted him by using his momentum against him, removing him from the fight for a second time.
After all was said and done, Juggernaut's attempts to make right were dashed and he was once again subservient to Cyttorak. And when left the X-Men once again, he warned Charles not to seek him out after his departure.