Colossus is a mutant with the superhuman ability to convert the tissue of his entire body into an organic steel-like substance, granting him superhuman strength and a high degree of resistance to bodily harm. This substance, resembling organic steel, is of unknown composition but appears to be analogous to osmium and to carbon steel. While in his armored state, Colossus possesses the same degree of mobility that he does in his normal form. However, due to the severe injuries Colossus endured on the battlefield, he is no longer able to convert to his human form without risk of death.

Piotr Rasputin was the younger brother of his nation's greatest hero, Mikhail, and served by his side as a Soviet Super Soldier. When Apocalypse's forces came to Moscow, Mikhail fought them off but was captured by Apocalypse. Colossus became leader of the Super Soldiers and faced his brainwashed brother in combat. Now a Horseman of Apocalypse, Mikhail destroyed all of Russia's super heroes except his brother. The severely beaten Colossus chose to dedicate his life to stopping Apocalypse and Mikhail and joined the X-Men.

During his first mission as an X-Man, Colossus battled Sabretooth for the life of a young mutant named Katherine Pryde. Colossus fell in love with her and the two would eventually marry. After years of fighting for Magneto's cause, Colossus lost the will to fight on a battlefield in Africa. He decided to retire from the X-Men. As a final favor to Magneto, he agreed to teach the next generation of mutants with his wife. Colossus briefly came out of retirement to lead rescue his future student Chamber from the clutches of his brother Mikhail. He was injured so severely from the battle that only Magneto's metal powers kept him alive. Afterwards, Colossus could no longer revert back to his human self without risking death.

The next time he came out of retirement was when Magneto offered Colossus and Kitty an important mission to save Colossus' sister, Illyana Rasputin, a mutant with reality traveling powers. Upon learning from Magneto that his sister Illyana Rasputin was in fact alive and not dead as she was mistakenly reported, Piotr became obsessed with the rescue of his sister and lead his team of young mutants on a suicide mission to break into the Core, Apocalypse's power source for North America located in Seattle, to retrieve her. While their pupils infiltrated the Core from the frontside, he and Shadowcat snuck into the core through the back using Kitty's ability to turn them intangible.

After they found her, Colossus ordered Shadowcat to take them and Illyana to safety. The three of them were made intangible and floated away, leaving the students behind. However, Colossus would later promise to return and help the kids. He would barely reach the entrance to the Core in time to stop the door from closing, but found the odds highly unfavorable as Husk was the only one of his students still left alive fighting against the entire infantry. In an act of cowardice, he locked her inside the Core without attempting to help and left her to die while saving his own life. He returned to his wife and sister and claimed he did everything possible to save them.

Colossus and Shadowcat delivered Illyana to Magneto in the heart of Apocalypse's base of operations. Colossus became enraged after Illyana was coaxed into entering the M'kraan Crystal with Destiny and Bishop in order to restore the natural timeline because he learned that, his sister would, in essence, be killed as she no longer existed in the reality they were trying to save. He rampaged towards the M'kraan Crystal in an attempt to follow his sister, but Shadowcat stepped in front of him, assuming he would stop. He didn't and Colossus crushed his wife. Shocked by the death of his wife, he was caught off guard and killed by Gambit before he could harm anyone else.