Hassan-i-Sabbah / Old Man of the Mountain
A legendary assassin known for the skull mask he wears. He's regarded as the original Hassan-i-Sabbah—the first to bear what would become a title rather than a name, emerging when the assassination cult solidified into the legend of the "Old Man of the Mountain." Whether he actually existed is debatable; no solid evidence survives, and supposedly none of the other 18 Hassans nor the cult's followers ever knew his true identity.
Fans often describe him as a figure of myth: a Hassan who hunts other Hassans, appearing when a generation's leader strays from the righteous path. According to legend, no cult member ever laid eyes on him and lived to tell about it. He's portrayed as both the origin of the "Old Man of the Mountain" title and as an inspector of the cult itself—a contradictory role he deliberately embraced.
His core conviction was simple but uncompromising: even if the cult's teachings align with God's will, their deeds remain human evil. He couldn't abide the contradiction of those sworn to protect God's law indulging in human corruption. That hypocrisy, in his view, was the ultimate blasphemy. When decay—literal or spiritual—infected the cult's leadership, it fell to him to deliver justice. He chose to fight openly with a large sword rather than hide behind assassination tools, making his role unmistakable: when someone sees the skull mask, they're already dead. After he exacts punishment, witnesses vanish from the world.
He's said to lurk in the cult's shadows even now, waiting for its final ruin—a living legend, the abyss of assassins, with no witnesses left to prove he existed.
Role in Fate/Grand Order: Babylonia
He appears initially as Ziusudra, an old bearded vagrant in Uruk, seemingly testing the protagonist's character once more. During the team's mission into the ruined city of Cuthah, he intervenes when Ereshkigal threatens to drag the protagonist's soul into the Underworld.
In the climax, after Tiamat is temporarily trapped below by Ereshkigal and Ishtar's combined efforts, he seizes the moment to strike. He slices off Tiamat's wings and imposes the Concept of Mortality upon her—a decisive blow that weakens the primordial evil. He announces he's surrendered his title as Grand Assassin for this one chance to strike against an Evil of Man, and fights alongside the protagonists in the final battle.
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