At some point in his life he was sold into slavery by the Romans. He was purchased and taken to a gladiator school (possibly in Capua) where he was trained to become a gladiator. Around the year 73 BC, Karros died fighting in one of the gladiatorial games and became Immortal. He was soon taught about his heritage by another gladiator, Obelix the Gaul. After a while, Karros met fellow Thracian, Spartacus, and joined him when he started a slave rebellion against Rome.Inspired by Spartacus, Karros became a warrior for the oppressed. For over 2,000 years, he has fought in many rebellions against many oppressors be it the Russians, the Mongals, the Turks and the Nazis during the second World War. By the mid 1980s he was in Africa fighting among guerrilla forces. By the early 1990s the conflict in the Balkans erupted and Karros joined in the fight against the Serbs. Unfortunately, Pyrrhic victories seemed to be his favorite kind.
In 1994, Karros and his assistant Mara Leonin arrived in the United States along with a peace negotiator named Father Stefan, to try and drum up support to stop the conflict. There was an assassination attempt on the preist which was foiled by Duncan MacLeod.Karros later met with MacLeod, whom he first met in 1867 during the Mexican Revolution. Although Duncan came to believe that Karros exhorted his followers to fight for reasons other than those in their best interests, he still greeted Karros as a friend. But soon Duncan came to learn that Karros was behind the assassination plot and was willing to do anything to sabotage the peace negotiations, his goal was the battle, not the victory.
When Mara also discovered Karros' treachery, she threatened to expose him. Karros responded by trying to kill her but failed. Duncan later met Karros at the docks, Paul attempted to convince Duncan to join his cause, but in the end faced the challenge. Despite the shock on Karros's face, Duncan took his head and quickening.
As Mara left to go back to the Balkans, MacLeod's friend, Charlie DeSalvo decided to go with her.
During the Mexican Revolution, Karros became suspicious of one of the peasant fighters who seemed to be spying on him. He tortured the man to death in an effort to learn his secrets but Watcher, Roberto Chavez died without revealing the existence of the secret society.