The Order of Assassins were a Nizari Isma'ili order that existed between 1090 and 1275 AD, founded by Hasan al-Sabbah.
During that time, they lived in the mountains of Persia and the Levant, and held a strict subterfuge policy throughout the Middle East, posing a substantial strategic threat to Fatimid, Abbasid, and Seljuk authority, and killing several Christian leaders. From Wikipedia
