Hassan-i-Sabbah, also known as the Old Man of the Mountain, is a prominent character in the Fate/Grand Order series. He is a legendary figure, renowned as the founder of the Order of Assassins. In the series, Hassan is depicted as a master of stealth and assassination, possessing mystical abilities and unparalleled combat skills. He plays a crucial role as a Servant summoned to aid protagonists in their quest to resolve various historical anomalies. Known for his wisdom and strategic mind, Hassan often serves as a mentor and guide, providing valuable insight and support to his allies.