Final answer:
The Abbasids were the group that opposed the Umayyads and were responsible for the overthrow and massacre of the Umayyad princes during the Abbasid Revolution in 750 CE.
Step-by-step explanation:
The group that opposed the Umayyads and played a crucial role in their downfall were the Abbasids. The Abbasids were descendants of Muhammad's uncle, Abbas ibn Abd al-Muttalib, and they led a revolutionary uprising resulting in the overthrow of the Umayyad Caliphate in 750 CE.
Supported by many non-Arab Muslims, who were dissatisfied with the Umayyad preference for Arabs, the Abbasids claimed the legitimacy of their rule on the basis of their direct relation to Muhammad's family. They orchestrated a massacre of the Umayyad princes in the event known as the Abbasid Revolution, successfully seizing control and establishing their own dynasty.