Was the medieval order of assassins a real thing?

Yep.

Explanation:

They were, sort of. The legends are based on the Nizari Ismailis—a breakaway group from the Ismaili branch of Shia Islam—that occupied a string of mountain castles in Syria and Iran from the end of the 11th century until the Mongol conquests in the middle of the 13th.

i believe that the answer to the question provided above is that  establishing smaller ethnic communities within larger communities  gives more chance for people to know each other more but this might lead to the  conflict between groups.