Wikipedia
The Baro-Bhuyans (spelled variously as Baro-Bhuinas, "Baro-Bhuiyan" etc.) were warrior chiefs and landlords ( zamindars) in medieval Bengal and Assam who maintained a loosely independent confederacy. In times of aggression by external powers, they generally cooperated in defending and expelling the aggressor. In times of peace, they maintained their respective sovereignty. In the presence of a strong king, they offered their allegiance. In general, they were in control of a group of villages, called cakala, and the more powerful among them called themselves raja. Baro denotes the number twelve, but in general there were more than twelve chiefs or landlords, and the word baro meant many. Thus, Bhuyan-raj denoted individual Bhuyanship, whereas Baro-Bhuyan denoted temporary confederacies that they formed. In Bengal they carved the land of Bhati into twelve administrative units or Dwadas Bangla
The system of Baro-Bhuyan confederacy is a relic of the erstwhile Kamarupa kingdom, that covered all of Assam, North Bengal and large portions of Bangladesh. The "parcelization" of power, which was an effect of settling North Indian adventurers, became prominent during the 9th century reign of Balavarman III of the Mlechchha dynasty. Whereas the central Kamarupa kingdom fragmented, the system of small chieftains remained. In Bengal as in Assam, the Baro-Bhuyans are found in regions within the traditional boundaries of the Kamarupa kingdom.
In Assam, the Baro-Bhuyans occupied the region west of the Kachari kingdom in the south bank of the Brahmaputra river, and west of the Sutiya kingdom in the north bank. They were instrumental in defending against aggressors from Bengal, especially in defeating the remnant of Alauddin Husain Shah's administration after 1498. They also resisted the emergence of the Koch dynasty but failed. Subsequently, they were squeezed between the Kachari kingdom and the Kamata kingdom in the south bank and were slowly overpowered by the expanding Ahom kingdom in the north. In Bengal, the Kingdom of Chandradvipa or Barisal was ruled by the royal Basu family, the last of which being Raja Rabindra Narayan Bose, who later fled to Kolkata during the partition of Bengal, as their position had already been reduced to that of a zamindar. The royal family thus, still lives in Kolkata.
These landlords did not belong to any particular ethnicity, religion or caste.