The rise of the Abbasid state and the pledge to al-Saffah
Abu'l-ʿAbbas ʿAbdullah ibn Muhammad, titled al-Saffah, was given the pledge of the caliphate in the mosque of Kufa — and with it arose the Abbasid state, which ruled the Muslim world for more than five centuries.
The Abbasid call set out secretly from Khurasan under the slogan "the accepted one from the family of Muhammad," led by Abu Muslim al-Khurasani, until the armies of the Umayyads crumbled before it, and the Abbasids entered Kufa in 132 AH.
In the congregational mosque of Kufa, Abu'l-ʿAbbas ascended the pulpit; the people pledged the caliphate to him, and he delivered his famous sermon in which he named himself "al-Saffah," announcing the start of a new era and a state that raised the black banner as its emblem.
Only months passed before the Umayyad state collapsed at the Battle of the Zab, and the Abbasid state was consolidated. It would move the capital to Baghdad and, in its golden age, witness the most brilliant era of Islamic civilization in knowledge and construction.
Note — differing reports on the date: The well-known view is that the general pledge was on 12 Rabiʿ al-Akhir 132 AH; some reports say the 13th, and some place it in Rabiʿ al-Awwal.