What Persian general took advantage of Nasr ibn Sayyar being preoccupied with a civil war? 
Nasr ibn Sayyar:  His last years were occupied by intertribal rivalries and uprisings, however, as the Caliphate itself descended into a period of civil war. Nasr ibn Sayyar:  Preoccupied with this conflict, Nasr was unable to stop the outbreak and spread of the Abbasid Revolution, whose leader, Abu Muslim, exploited the situation to his advantage. Abu Muslim: Abu Muslim Abd al-Rahman ibn Muslim al-Khorasani or al-Khurasani (Arabic: أبو مسلم عبد الرحمن بن مسلم الخراساني‎ ‎ born 718-19 or 723-27, died in 755), born Vehzādān Pūr-i Vandād Hormoz (Persian: وهزادان پور ونداد هرمزد‎ ‎ ), was a Persian general in service of the Abbasid dynasty, who led the Abbasid Revolution that toppled the Umayyad dynasty.
Abu Muslim