Colossus died in Uncanny X-Men #391.

Name: Colossus Real Name: Piotr Rasputin Mutant Power: Ability to change his form into a practically invurnable, organic steel form. Biography: Colossus was born and raised from Russia. He was recruited by Professor Charles Xavier to join the "second Genesis" X-Men team. At the time, the X-Men were composed of Cyclops, Marvel Girl, Iceman, Angel, Beast, Lorna Dane and Havok. Those 7 X-Men were captured and only Cyclops managed to escape. So, along with the mutants, Sunfire, Storm, Wolverine, Nightcrawler, Thunderbird, and Colossus, Cyclops and the second Genesis rescued the captured X-Men. After the rescue of the captured X-Men, 13 X-Men remained. Lorna Dane, Havok, Angel, Iceman, Marvel Girl and Beast agreed that there training was complete and they left the X-Men to live their own lives. So, the second Genesis team, including Cyclops, stayed as X-Men. Soon after, Sunfire left the X-Men, Thunderbird died because of his foolish ego and Marvel Girl returned and became the Phoenix. Long after, Colossus's little sister, Illyana was taken to a dimension called "Limbo" where she aged at a rapid speed and became a highly trained sorcerress. Some time later, Illyana had contracted the legacy virus-- a deadly virus set out to kill mutants. Once the legacy virus is contracted, it is terminal. Illyana died and the memory of his sister's death had haunted Colossus forever. When the X-Man, Beast (in Ucanny X-Men #391), discovered the cure for the legacy virus, he realized that it could only be ended with a mutant sacrafice. So, in honor of his sister's death and all the others that fell victim to the virus, Colossus injected the cure inside his body. Slowly, Colossus's life fled from him. Dr. Cecilia Reyes tried to stop Colossus death, but it was too late. Colossus made a noble sacrafice and the legacy virus had been killed...for good.