Fate Zero is as complicated as it can get. Primarily because it’s not the First Grail War we as an audience witness. We don’t get the gist of it initially. Another reason is the convoluted ‘Fate’ lore. There are so many strings attached to it that one doesn’t know where it actually started. And due to that some questions always remain unanswered. Although many questions need to be answered, today we are going to discuss how Gilgamesh survived the Fuyuki Grail War and became a human.
After knowing that the Grail is corrupted and only wants the destruction of humanity, Kiritsugu ordered Saber through his remaining command seals to destroy it for good, which she did unwillingly. But that only led to the destruction of the Lesser Grail.
Meanwhile, when Kiritsugu rejected the Holy Grail, it allowed Kirei to make a wish instead and his wish was the destruction of humanity so the Grail spiller out its contents, the black mud. Think like this, there’s a cup filled with some substance and if you destroy it, the cup certainly gets shattered but the substance doesn’t. The substance here was the 60 years’ worth of preserved magical energy which was meant to fulfill wishes. Along with that, it was corrupted by Angra Mainyu so it was destined to bring forth destruction only.
But how did Gilgamesh survive all of that and become human?
The black mud that oozed out of the cup should have either killed him or cursed him at the very least but it didn’t and rewarded him with a tangible body and also gave life to Kirei while cursing Emiya for his whole life.
First things first, Gilgamesh was an Archer level servant, but a servant like any other. He needed mana to even exist in the human world. The mana was provided by his master, in this case, Kirei. The mana flows from master to servant. So when the black mud got spilled out of the cup, the 60 years’ worth of mana gave him a tangible body because that’s what being in concentrated magic does to a heroic spirit. There was more than enough mana to grant Gilgamesh a human body, bring Kirei back and also to destroy the entirety of Fuyuki City.
So, in short, Gilgamesh survived and became human at the end of Fate:Zero because the excessive convoluted mana that spilled from the Grail gave him a tangible physical body.
Also, there’s another theory where Lancer is involved. After the unfair battle with Saber and Kiritsugu, when he is on his verge of death, he curses the Grail and humanity.
This might also be the reason for the destruction of Fuyuki City and as Gilgamesh was this arrogant, prideful and egotistical heroic spirit who wanted to reshape humanity, he was given a chance, an opportunity to fulfill his wish. Reshaping humanity under his rule once again could be one of the reasons why Gilgamesh decided to stay back in this world.
The same goes for Kirei, as he was nihilistic and sought destruction, he was revived in the process too.
Gilgamesh is the great half-god, a half-human king born from the union between the King of Urum, Lugalbanda, and goddess Rimat-Ninsun. In addition to that, he was destined to achieve feats that humanity wasn’t able to do before. He was the first King the world ever saw and proclaims himself as ‘King of Heroes’ implying that he is the King of all Heroes, the origin of myths and models which the heroes were based.
In Fate Zero, he was an Archer class servant of Tokiomi Tohsaka in the Fourth Holy Grail War. Later on, he forms a contract with Kirei Kotomine as he piqued his interest and also because Tohsaka was not intriguing enough for him. He helped Kirei to embrace his sadness and gave it meaning so that he finds the solace he has been longing for ages.