Fate/Grand Order Concept Masterline Series statuette 1/6 First Hassan Bonus Version 56 cm

An assassin that wears a skull mask.
Regarded as the first Hassan Sabbah, who was born when the name “Old Man of the Mountain” changed into a title, after the assassination cult became established as assassins.
However, it has been said that neither the other 18 Hassans nor the followers of the cult knew this individual’s true identity and that no evidence that he truly existed could be found anywhere.
He is a legendary assassin, said to appear to take the head of a generation’s Hassan-i-Sabbah when they stray from the path.
In other words, a Hassan that kills other Hassans.

Supposedly there was not a single eye-witness within the assassination cult who saw him, which is not surprising - one’s life would be forfeit from the moment he lay eyes on this individual.
At the same time that he was the origin of the “Old Man of the Mountain”, he also chose the life of an inspector that convicts the corruption of the cult.
Even if the cult is righteous according to the teachings of God, that does not change the fact that its actions are evil as human beings. Therefore, this individual could not forgive that the cult could become degenerate.
For those who protect the teachings of God to wallow in human cravings.
That in itself is the greatest blasphemy against God.

The corruption of the cult is, namely, the depravity of its top - the “Old Man of the Mountain”.
Regardless of whatever that is a depravity of the mind or of the skills, the name of “Old Man of the Mountain” is not bestowed to one who has decayed.
Decay is, namely, death. The severing of one’s head is both a pardon for his sins and a hope to be entrusted to the next “Old Man of the Mountain”.
In order to provide just cause to the contradictory role of assassinating assassins, this individual chose not concealed weapons・assassination tools, but to openly fight with a large sword.
When someone catches sight of this skull, said person has surely met his end.
After he has delivered punishment upon the fool who has failed to keep the cult’s dogma, those who have seen the figure of the skull swordsman disappear from this world.
Having consigned many assassins to oblivion, he is said to lurk in the darkness of the assassination cult until the day of its ruin.
A living legend, the abyss of the assassins without a single eye-witness.
Such is this skull swordsman.

Role in Fate/Grand Order: Babylonia:

Initially taking the appearance of an old, bearded vagrant and assuming the name of Ziusudra, he awaits the protagonist in the city of Uruk -- apparently to again test the protagonist's character. Subsequently, during the protagonists' exploratory mission to the abandoned city of Cuthah, he intervenes when the protagonist's soul is drawn into the Underworld by Ereshkigal.

In the finale of the Singularity, the protagonists are able to temporarily trap Tiamat within the Underworld by the assistance of Ereshkigal and Ishtar -- aiming to utilize the Anti-Divinity property of the territory of the Underworld to weaken her. In the midst of Tiamat's attempt to escape to the surface, the Old Man of the Mountain intervenes by slicing off Tiamat's wings and imposing the Concept of Mortality upon her -- stating to the protagonists that he has removed himself of the title of "Grand Assassin" for the opportunity to strike against an Evil of Man. He participates in the final confrontation with Tiamat.

(Source: TYPE-MOON Wiki)