Actually the only reason why king Hassan lost is saint graph was because as a Grand servant he was not supposed to to interfere him slapping the concept of death on her and nothing being ordered to do it is the reason why he lost it plus Grand servants can't just be summoned they need to lose their saint graph in order to be summoned.
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slapping the concept of death was to provide chaldea a way to "kill" her but since she started to fly and tried to escape the underworld he went to slow her down and cut her wing.
Tiamat's seal as nothing to do with her holding back it's actually her performing.. The correct term would be Self-flagellation I don't see where you got the idea that Tiamat is holding back when it's clearly stated during the event of her arrival she basically lost control and went bat shit insane.
Also Merlin: Hm. We delayed her with two goddesses. Made a hole to the Underworld using Uruk as bait. Restrained her with the Chain of Heaven, inflicted the punishment of the Underworld, and made flowers which are merely beautiful. Gudao, and you heroic spirits. You truly did much to get where you are now. But it isn't enough. It still doesn't know fear. It doesn't know its natural enemy. It doesn't know the death which is him.
Mashu: Him? Is there someone else who can help us?
Merlin: Indeed there is. A very important person. Well then, just whom was he called by? It wasn't by King Gilgamesh. Nor was it by the Grail of the King of Magic. Need I say it. It was by you, Gudao.
He said he would throw away his Grand rank in gratitude to you. And the foe is an enemy of humanity, a Beast. The conditions by which he could appear in this land had been in place from the start! Your battle, all of it, had meaning.
Dr. Roman: …Could it be? The reason why Gudao's bioreadings disappeared in the underworld before was because…
Merlin: That's right. Now behold the sky, o primordial sea! For that is where your grim reaper stands!
Grand Battle Music
Ziusdra: …There cannot be life without death, and it is because there is death that there can be life. The eternity thou claims is not progress but slumber. Beast of disaster, evil born by humanity. The love that brought thee to seek regression is what casted thee aside.
Tiamat: Aaaaaa----AaAa, AAAAAAAAAAAAAAAAA
Ziusdra:
I need not the rank of Grand, but in my blade now dwells the proof of the strongest.
Old Man of the Mountain: Thou may be a god that has fallen to a beast, but thou art still the primordial mother, so I must reveal my name. From the abyss of the ethereal valley, I came to deliver the dark death. I am the old man of the mountain, Hasan Sabbah. The Evening Bell has called thy name. Let those wings be deprived from you under the mandate of heaven!
Slash
Dr. Roman: The pattern of the Beast's spiritual foundation has changed… Incredible…he didn't just cut off Tiamat's wing, he even added the "concept of death"!
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 as a guest in the second-to-final confrontation with Tiamat.
Him losing is saint graph did not make him weaker he just lost the title that's all because grand servant unless told by the counter force, are not allowed to interve until they are told to do so but King Hassan went against that making him lose his saint graph y but not his powers.