The Hashashin were a sect of the Nazari derived from the Ismaili sect of Shiite Islam. They emerged under the leadership of Hassan-i Sabbah in1090 after a series of internal power struggles within the Shiite branch of Islam. Hassan led them in the conquest of the strategic fortress of Alamud in Persia, just south of the Caspian Sea.
Lacking the numbers to mount conventional challenges against their more powerful neighbors, they developed a cadre of skilled assassins who infiltrated enemy forces and waited for orders to strike. They were essentially sleeper agents. Their name comes from unfounded rumors that believers used hashish. The word assassin is also derived from Hashashin.
In The Templar Concordat, this origin is what the present day Templar Marshall refers to when he cautions that the Hashashin are just as old and clever as the Templars.