The Old Man of the Mountain bears the title of the first Hassan-i-Sabbah and founded the Hashashin assassins. His true identity remains unknown, proven only by his execution of any Hassan who strays. He accepted the moniker "King Hassan" from Ritsuka Fujimaru. As the inaugural Hassan, he served as the order's origin and final arbiter, residing in obscurity until its destruction. He inhabited the Holy Shrine of Azrael, a mausoleum north of Jerusalem symbolizing the boundary between life and death, accessible solely to those whose lives have ended. His appearance features a skull-shaped helmet concealing his face, revealing only empty blue pupils. He wears thick purple and black armor adorned with skeletal motifs, including a cuirass displaying a skull with blue flames emanating from its eye sockets. Pauldrons on his shoulders similarly emit blue flames, complemented by a worn mantle and cape. Standing approximately 192-220 cm tall, he wields a greatsword as his primary weapon, diverging from conventional assassination tools. Personally stern, he adheres to an unwavering faith-based code. He exhibits low tolerance for impoliteness, initially attacking uninvited shrine visitors. Yet he shows compassion, willing to hear pleas for aid under strict conditions—demanding a Hassan's life as recompense, viewing reliance as dishonorable. He perceives life's end as divine salvation and enacts merciless judgment, often communicating through his skull mask's glowing eyes rather than speech. He displays no interest in other Heroic Spirits but relentlessly criticizes successive Hassans, frequently demanding their heads for perceived failures. His relationships reflect his role as enforcer of the Hashashin's tenets. He criticizes Hassan of the Cursed Arm for grafting a demon's arm, Hassan of Serenity for her poisonous isolation, and Hassan of the Hundred Faces for internal discord. He respects death-associated figures like Ereshkigal, acknowledging her virtues despite differing beliefs, and approves of Nitocris' reverence for the deceased. In the Camelot Singularity, he tested intruders at his shrine and aided Chaldea against the Lion King's forces, stipulating Hassan of the Cursed Arm's life was forfeit for requesting help. He intervened in battles, countering Gawain's advantage by shrouding the sun before retreating once his role was fulfilled. During the Babylonia Singularity, he first appeared disguised as the sage Ziusudra to assess the protagonist. He later rescued them from the Underworld and confronted Tiamat, severing her wings and imposing the concept of death upon her—an act requiring him to relinquish his status as Grand Assassin. He stated serving Chaldea necessitated abandoning the Grand title, reserved for humanity's saviors. His Saint Graph retained Grand-class power briefly after this transition, enabling him to weaken Tiamat before reverting to a standard Servant level. His abilities center on death manipulation. His Noble Phantasm, Azrael, is a greatsword inflicting instant death, symbolizing his fusion with the boundary between life and demise. He wields skills like Evening Bell, reducing enemy resistance to instant death and enhancing his Buster-based attacks, and Protection of the Faith, granting debuff resistance, healing, and defensive boosts. Unique traits include immunity to instant death and charm effects, plus a passive chance to inflict instant death with normal attacks. Unlike typical Assassins, he forgoes stealth for direct combat, reflecting his duty to eliminate wayward order members.