The Dark Age of Assamese Language is the time-frame during which Bengali Language eclipsed the Assamese.
This time-frame is about 37 years long, from 1836 until 1873. During British India, the Bengali Language was imposed over Assamese as the British took over Assam. The clerical and technical workers that they brought were Bengali, imposing Bengali as the medium of instruction in schools and colleges and for all official purposes.
Nathan Brown, an American Baptist missionary to India, worked for the restoration of the Assamese language to avoid it being overtaken by the Bengali language completely. In order to restore the Assamese Language, he took the matter to the British India administration of the time. Eliza Brown was Nathan's wife and she had been partner of her husband's mission.