Question: What Persian general took advantage of Nasr ibn Sayyar being preoccupied with a civil war? 
Sentence (from "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.
Relevant evidence: Yes