Dark Age of the Assamese language
The Dark Age of the Assamese language is a 37 year long time-frame, from 1836 to 1873, during which Bengali is considered by the Assamese-speaking people to have eclipsed the Assamese language. During British India, the Bengali language was introduced in Bengal by replacing Persian as the official language, as the British took over Assam.
Source: Wikipedia — Dark Age of the Assamese language (CC BY-SA 4.0)