Old Man of the Mountain (Assassin)
The Old Man of the Mountain (, ), is the expression used by Marco Polo in a passage from Book of the Marvels of the World, to indicate Hassan-i Sabbah, the grand master of the Order of Assassins, who took refuge in Alamut Castle. It later became a common name used by the Crusaders. Subsequently, this nickname was given to various Isma'ili successors of Hassan, in Syria, particularly, for example Rashid al-Din Sinan, the da'i (missionary) and a leader of the Syrian branch of the Nizari Isma'ili state. References

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Order of Assassins
Marco Polo
Nizari Ismaili state