BIOGRAPHY - Page 3
Colossus faced some of his own personal demons when the Marauders launched their massacre on the Morlocks. Fighting hard to save as many Morlocks as he could, Piotr once again crossed the line and killed the assassin Riptide, snapping his neck after he had almost killed Nightcrawler. [Uncanny X-Men (1st series) #211]
The near constant battle left Colossus' metallic state drained, the bio-energy that powers his body leaking out from invisible ruptures in his steel form, apparently caused by Riptide’s throwing stars. 
Piotr fought the pain as long as he could but suddenly collapsed in the mansion. Detailed examination revealed the problem and Magneto attempted to stop the hazardous process with his magnetic powers. He succeeded but ended up paralyzing Piotr and trapping him in his organic steel state. [Uncanny X-Men (1st series) #212]
Colossus was flown to Muir Island for treatment and, over the following weeks, he overcame his paralysis. 
A side-effect of Magneto's treatment was that Piotr's metallic mass increased to a certain degree, augmenting his physical size and strength considerably in his armored state. As a detriment, though, he now required a considerable act of concentration to revert to his human form and any amount of distraction could make his return to his steel form. [Uncanny X-Men (1st series) #215, 225]
When the X-Men came under assault by Freedom Force and the Adversary in Dallas, Texas, Roma - Guardian of the Omniverse - appeared to Colossus in disguise and sent him to fight by his comrades' side. Colossus turned out to be Roma’s secret weapon, as his unexpected arrival threw the Adversary off guard and his metal form was anathema to magic. However, to fully defeat the evil trickster, Forge needed to transform the willing X-Men into energy and hurled them as a weapon to banish the Adversary from Earth. Roma rewarded the X-Men’s noble sacrifice by resurrecting them and the team decided to remain underground, letting the world believe them dead. They soon appropriated an abandoned town in the Australian Outback from a gang of cyborgs called Reavers. The X-Men made this their new base of operations and relied on their ally, Gateway, to teleport them anywhere in the world. [Uncanny X-Men (1st series) #226-227]
Though this team of X-Men was mostly new to Piotr, he soon began to mesh well with his teammates once more. However, it troubled him that he had to let his family believe him dead. When he had dreams of Illyana being in danger, 
he wanted to leave and make sure that she was all right, even if that meant exposing the X-Men’s secret. Gateway teleported him to Illyana, who, at the point, was busy fighting demons in Limbo and had almost finished casting a necromancy spell to summon the dead back to life. Due to the coincidence of timing, she believed her brother to be a ghost and Piotr decided to use this to maintain his cover while helping Illyana at the same time. Piotr was somewhat disturbed by Illyana’s changing appearance - she had grown horns, hooves and a tail - but he stood by his sister nonetheless. When she voiced concerns that she was turning evil, he reminded her that the very fact that she was wanting and trying to do good proved that she was not. When the battle was over, Gateway took him back to Australia. [Uncanny X-Men (1st series) #231]
Another adventure soon led the X-Men back to the Savage Land, or what remained of it. Instead of the prehistoric jungle, they found only ashes, as the Savage Land had been destroyed in a disaster caused by the alien Terminus. Its people, however, had escaped to an adjacent dimension, where time moved somewhat faster. Colossus was reunited with Nereel who now was the chief of her tribe and also had an approximately six-eight year old son. Piotr asked about the child’s father, wondering if he had perished in the fire, only to learn that he had left Nereel long before. He failed to notice that Nereel was obviously speaking about himself, as the boy was even named after him, Peter. Only Colossus‘ teammate, Longshot, deduced the truth but Nereel swore him to secrecy. With the help of the High Evolutionary, the Savage Land was restored and the X-Men left, again trying to maintain their cover. [Uncanny X-Men Annual #12]
Isolated in the Australian Outback, the X-Men had only each other for company. While several of the others started relationships, like Dazzler and Longshot or Havok and Madelyne, Colossus became especially close to Psylocke, who even agreed to pose for some of his paintings under the promise that she would get to keep the portrait. [Uncanny X-Men (1st series) #240]
When Inferno descended upon Manhattan, the X-Men were right in the thick of it. The tangible demonic aura infecting the city began to change the X-Men as well. Only Colossus remained unaffected, presumably due to a combination of his pure spirit and his metal body, a natural anathema to black magic. Unable to help his teammates, Colossus wandered off on his own, only to stumble into a confrontation between S’ym and the Darkchylde.
At first, Piotr had no idea that the demonic figure clad in an Eldritch armor covering it from head to toe was none other than Illyana. His shock and horror upon learning this was so great that Illyana teleported away in shame, though having seen her brother again helped her to focus. She cast all her magical might into one spell, sacrificing all her magical might. New York was restored but at a cost. When the others found the scorched Eldritch armor, they thought her dead but suddenly a voice from the inside was heard. Colossus ripped it apart to discover the little child Illyana within, back to the six year old girl she was supposed to be. [New Mutants (1st series) #73] Colossus trusted the New Mutants in returning her to Russia while he and the X-Men departed for Australia.
In the following weeks, the X-Men slowly began to drift apart. Some were apparently killed, while others left of their own accord. With only four X-Men remaining, Psylocke was convinced that they had only one option left when she sensed the Reavers closing in to attack the exhausted team. Betsy suggested to escape through the Siege Perilous, a mystical portal which judged all who entered it and sent them to new lives. After a short discussion, it was settled, though upon entering the mystic doorway, Piotr wondered if he would see Betsy again in his new life. [Uncanny X-Men (1st series) #251]
Piotr emerged in a loft in Manhattan, naked and virtually amnesiac. The loft belonged to Phillip Moreau and Jenny Ransome, Genoshan emigrants, who did not recognize Piotr from their earlier meeting as they had only seen him in armored form. 
Utilizing his creative talent, Piotr set up a new life for himself as an American painter in Soho, calling himself Peter Nicholas and soon became quite famous. Piotr also became romantically involved around this time with Callisto, who had been altered by Masque with the features of a beautiful model. "Nicholas" was almost completely unaware of his powers and his past with the X-Men, though he did encounter a number of faces from his past including X-Factor and Banshee and Forge, the latter two of which were specifically searching for the missing X-Men. Since Piotr seemed happy in his new life, they left him alone. If they would need him later on, they would know how to find him. [Uncanny X-Men (1st series) #259-260, X-Factor (1st series) #54, Uncanny X-Men (1st series) #262-264]