Abu Muslim was the leader of the revolutionary movement in Khorasan against the Ummayyad Dynasty. He helped the Abbasids come to power.
He was born a mawali (non-Arab Muslim). He met an Abbasid agent while he was in prison in 741 A.D. After release from prison, he was sent to Khorasan (745-46 A.D.) to start a revolt. He recruited as many discontented men as he could and overthrew the last Umayyad caliph, Marwan II in 750 A.D. As a reward, he was made governor of Khorasan. The second Abbasid caliph, al-Mansur saw him as a threat and had him executed.
Further Reading:
The Caliphate: Its Rise, Decline, and Fall - From Original Sources