The term refers to a substantial number of individuals who engage in organized, sustained activity in order to bring about some social change or to oppose some social change and who, to some degree, utilize noninstitutionalized forms of political action.

Social movement

Explanation:

Social movement refers to the gathering of people who are engaged or involved in organized activities and make sustained and organized efforts in order to show their support to social change or their opposition in order to show their resistance to some change in the society in the long term.

It sometimes make use of non institutionalized political action upto some extent.

imperial crisis leads to civil war.

roman forces withdraw from britain as goths attack rome.

odoacer topples romulus augustus

huns attack the empire and drive german peoples ahead of them.

sounds good to me mate!