Marvel history was made in the 1990s when the Juggernaut's shocking defeat ushered in the reign of Onslaught, a major X-Men villain who reshaped Marvel's entire reality. However, while Juggernaut's terror was the perfect introduction to such a powerful being, things behind the scenes weren't at all what they seemed,
In Uncanny X-Men Vol 1 #322 Onslaught is first hinted at as the famously unstoppable Juggernaut is hurled across the country by the new villain, only to be discovered by the X-Men. Onslaught isn’t even seen in the issue, instead lurking in the background of various X-Titles for the next year, operating via proxies and psychic attacks before finally revealing itself, becoming the villain of the titular Onslaught event in 1996. Onslaught is a psychic being created from the worst parts of Charles Xavier’s psyche, which fused with Magneto’s own evil when Charles invaded Magneto’s mind in X-Men Vol 2 #25 by Fabien Nicieza and Andy Kubert. This being of pure negative emotion may be a huge threat, but when it first attacked Juggernaut, Onslaught didn’t actually exist yet.
In Comic Creators on X-Men by Tom DeFalco, Uncanny #322 writer Scott Lobdell reveals that when #322 (by himself and Tom Grummett) was written, he had no idea who Onslaught was, and had only come up with his name and the idea of him being stronger than the Juggernaut. The reason that most of the hints towards Onslaught’s identity were vague is because the writers themselves were making up the villain as they went, with the story even playing with Xavier having turned evil for a while before settling on Onslaught as a separate being. After freaking out about his defeat, Juggernaut utters the single name "Onslaught," but at the time he said it, that name is about all that existed of the character. Lobdell states:
I just thought it was a cool way to open a story. Imagine someone so strong that they could hurl Juggernaut across the sky! I ended up doing that opening sequence, but I still didn't know who Onslaught was.
Onslaught Barely Existed When He Beat Juggernaut
Onslaught was later retrofitted into prior comics: in X-Men Vol 1 #106 by Chris Claremont Bill Mantlo, Dave Cockrum and Bill Brown, as well as in Mantlo, Claremont, and Jackson Guice’s X-Men and the Micronauts #1. Similarly, the mysterious X-Traitor of Bishop’s future was retconned as Onslaught, when many prior hints had suggested it was a future Gambit. This welding of previous continuity onto Onslaught is inelegant, but it established a precedent for Charles’ psyche to split and act independently for evil. But at the point Juggernaut gave his ominous warning, not only was Onslaught just a name, but all this prior 'backstory' hadn't yet been retrofitted to involve him.
Juggernaut's Biggest Loss Came from His Own Power
It was later established that Juggernaut was Onslaught's first victim because of his connection to Charles Xavier (the two are step-brothers), but the real reason is that he was so strong, it was incredibly impactful to introduce a new big bad by having them beat him. Juggernaut's iconic loss only happened because he was seen as so powerful, and yet the Onslaught event grew to be so powerful, it became Cain Marko's most famous moment. Embarrassingly for Juggernaut, however, at the time he was actually beaten down by Onslaught, the villain was literally nothing more than a name.