The Assassins (Arabic: حشاشين Ḥashshāshīn, also Hashishin, Hassassin, or Hashashiyyin) were an order of Nizari Ismailis, particularly those of Persia and Syria that formed around 1092. Posing a strong military threat to Sunni Seljuq authority within the Persian territories, the Nizari Ismailis captured and inhabited many mountain fortresses under the leadership of Hassan-i Sabbah. The modern word "assassin" is derived from their name.
They smoked hashish to fortify themselves for their murderous missions, didn’t they? Hence the name?