If the Umayyads were the first ever Arab Empire, then the Abassids can be rightfully hailed as the Pioneers in Multicultural empirical reign.
The contrast between the governance and constituency of each empire couldn't have been any more dissimilar, and for good reason too. While the Umayyads maintained a distinct and strict central regime characterised by a prominent Arabic heritage, the Abassids adopted a more regional and decentralised Franchise which embraced and incorporated various languages and cultural influences. In fact, the Abbasid Revolution was in many respects a revolt masterminded and spearheaded by the discontent 'Ajjam (non-Arabs) against what was perceived to have been an elitist Arab-Supremacist regime under the reigns of the Umayyad Caliphs.
In sharp contrast to this, the Abassid Caliphate implemented a diversity policy that was so inclusive and accomodating that it very nearly undermined the reign of the Caliphs and almost deprived them of central authority altogether.
(𝘚𝘪𝘥𝘦𝘯𝘰𝘵𝘦, 𝘵𝘩𝘦 𝘜𝘮𝘢𝘺𝘺𝘢𝘥𝘴 𝘩𝘢𝘥 𝘢 𝘮𝘶𝘤𝘩 𝘴𝘵𝘳𝘰𝘯𝘨𝘦𝘳 𝘴𝘦𝘯𝘴𝘦 𝘰𝘧 𝘈𝘴𝘢𝘣𝘪𝘺𝘺𝘢 𝘢𝘴 𝘱𝘦𝘳 𝘐𝘣𝘯 𝘒𝘩𝘢𝘭𝘥𝘶𝘯'𝘴 𝘵𝘩𝘦𝘰𝘳𝘺 𝘰𝘯 𝘴𝘰𝘤𝘪𝘢𝘭 𝘤𝘰𝘩𝘦𝘴𝘪𝘰𝘯 𝘢𝘯𝘥 𝘵𝘩𝘦 𝘳𝘪𝘴𝘦 𝘰𝘧 𝘦𝘮𝘱𝘪𝘳𝘦𝘴)
As a matter of fact, such was the influence of the 'Ajjam (Non Arabs) within Abbasid realms that one particular family virtually ruled the empire from behind the scenes for a considerable period of time.
Enter the 𝗕𝗮𝗿𝗺𝗮𝗸𝗶𝗱𝘀, 𝗜𝗿𝗮𝗻𝗶𝗮𝗻 𝗕𝘂𝗱𝗱𝗵𝗶𝘀𝘁 𝗰𝗼𝗻𝘃𝗲𝗿𝘁𝘀 𝘁𝗼 𝗜𝘀𝗹𝗮𝗺.
They were a priestly Buddhist family of Iranian origin who occupied the region of Balkh, in Khorāsān. The Barmakids derived their name from the high Priestly cast of Barmak, which was a post recognised within the Buddhist temple of Nawbahār.
The family converted to Islam in the 7th century when Balkh fell under Muslim reign during the Caliphate of Uthman Ibn Affan. The Baraamikah were highly educated, trusted and respected members of society, becoming Finance Ministers, Regional Governors and Viziers to the Caliphs. They had also been respected by the Umayyad administration during the reign of Caliph Abdul Malik Ibn al-Marwan however their true potential was realised under the Abassid Caliphate.Khalid al-Barmaki was the first of the Baraamikah to serve the Abassid Caliphs, beginning with Al-Saffah and Al-Mansour.
His influence and prominence enable him to cement himself in the empire's administration allowing his family to grow ever more influential and powerful. At the height of their power, the Baraamikah even controlled the Caliph's elite military regiment in addition to keeping their own private army (Mamluks).
Although we do not know how this family was eventually dismissed from the Caliph's court altogether, what we do know is that their influence and affluent lifestyle did arise concerns from Harun Al-Rasheed during his reign. There are many elaborate theories and accounts detailing how they were ultimately expunged from their powerful posts and role within the Abassid Empire.
You may be interested to know that the Barmakids inspired one of Disney's most villainous characters. 𝗝𝗮'𝗳𝗮𝗿 𝗜𝗯𝗻 𝗬𝗮𝗵𝘆𝗮 𝗔𝗹 𝗕𝗮𝗿𝗺𝗮𝗸𝗶 was the real life vizier who inspired the infamous Jaafar from Disney's Aladdin franchise.
Jaʻfar ibn Yahya Barmaki was a real person (though not as villainous as portrayed by Disney) and was so close to the Caliph Haroon Al-Rasheed that they two were considered brothers, having virtually grown up together, raised by Yahya Al Barmaki, a very wise and powerful advisor to Caliphs Al-Saffah and and Al-Mansour.
The Barmakid family converted into Islam after having been a prominent Buddhist family in Balkh (within the Nava Vihara Buddhist Order). Over the centuries the legendary influence of the Barmakid family inspired eventful tales of the conniving evil Vizier who was plotting against the Caliph in order to seize power for himself and marry the Princess (which ironically was reportedly going to be the case with Ja'far and Haroon's sister Abbasa bint al-Mahdi).
Sadly, the Barmakids were rounded up one night and swiftly executed by Caliph Haroon Al-Rasheed for no apparent reason known to historians. Their prominence was short lived however they remained In folklore and cultural tales (Arabian Nights, Aladdin etc).