Jorge Ivan Laverde admitted to directly killing a hundred people and ordering the killings of 4000 others as a commander in the extreme right-wing AUC militia, which was disbanded in 2003.
A Colombian court initially sentenced Laverde to 40 years in prison after finding him guilty of murder, torture, acts of terrorism, and the displacement of population.
But the sentence was changed to eight years, the maximum penalty allowed under the "Justice and Peace" process aimed at healing the wounds of past fighting.
The right-wing paramilitary groups were initially set up in the 1980s and by the following decade had seized large swathes of territory, ostensibly to protect landowners from left-wing guerillas.
About 30,000 militiamen demobilised when the groups were dismantled in 2003, according to official figures, but about 9000 are believed to have since taken up arms, with many involved in drug trafficking.