For as long as he has been part of the Marvel Universe, Colossus has also been a fan-favorite X-Men mainstay. While he's served on multiple X-teams over the years, he stepped away out of Professor Xavier's purview to work alongside another famous mutant leader as part of Magneto's Acolytes in the '90s. Unfortunately, this only set Colossus and the rest of his allies on both sides of the aisle up for a world of hurt, even if he did manage to make everyone involved a little bit better along the way.
The Legacy Virus was one of the scariest threats to mutantkind in the early '90s, and when it claimed the life of Piotr's de-aged younger sister Ilyana Rasputina, the New Mutants' Magik, the foundation of the X-Men itself was shaken to its core. The loss of his sister, coupled with a brain injury that had trapped him in his armored form, led Colossus to question just how much good Xavier's mission was actually doing.
And in 1993's Uncanny X-Men #304, by Scott Lobdell, John Romita, Jr. and many others, Colossus left the X-Men and joined the Acolytes at Illyana's funeral.
Despite being a dedicated member of the far more militant team, Colossus never truly connected with Magneto's teachings, either. In Fabian Nicieza and Andy Kubert's X-Men #25, this was made most evident when the X-Men Blue Team led by Cyclops infiltrated Magneto's Avolon headquarters stationed in orbit on Asteroid M. As the chief security officer, Colossus was the one to see the alarms when his former teammates had arrived, just as he was the one to eschew those same warnings and allow the X-Men to complete their mission against the Acolytes.
The X-Men's invasion of Avalon quickly led to a full-blown battle within the space station between the two mutant teams, one which Colossus managed to almost entirely avoid. While Colossus stayed out of the fray, Magneto and Wolverine both inflicted nearly lethal damage upon one another. When the dust had finally settled, Colossus chose to stay behind with the Acolytes, both to look after Magneto's recovery as well as to continue his personal search for answers. For a time it seemed like this might have even been possible, but 1993's Excalibur #71 gave Colossus a new perspective on both sides of the battle he had been fighting. His longtime romantic interest Kitty Pryde was instrumental in helping free Colossus from the effects of the physical trauma he had suffered. And with that renewed clarity came the acknowledgment that both Xavier and Magneto's schools of thought were ultimately the same in every way but their execution. With that, he once again decided to stay with the Acolytes, this time thinking that maybe he could be the force for change that they needed to be a better version of themselves.
Having grown disillusioned in their methods and motives, Colossus eventually found a place for himself alongside Kitty Pryde and Nightcrawler once more as part of Excalibur in 1995's Excalibur #92, by Warren Ellis and Casey Jones. This would be just another stop along the way for Piotr Rasputin, though not an unimportant one in itself. Still, few periods in the character's history have been quite as impactful as his time with Magneto's Acolytes.
Even if they weren't the most action-packed moments of Colossus' superhero career, they did reshape who he was as both a person and hero, with Colossus even admitting that the culmination of his experiences had turned him from an abject atheist to a believer in something, even if he didn't quite know what that was just yet.