Fuyuki was burning, transformed into a modern Golgotha by the mud spilling forth from the hole in the sky. In the middle of this hellish landscape, was a boy, in the process of being consumed by the black flames of humanity's sins. The fire, born from an everlasting amount of malice and the Third Magic, sustained itself not merely on the physical, but on the spiritual as well. Wood, metal, suffering, Origins … the flames made no distinction between these as it burnt them for fuel.
The boy walked on. He was in a state of tabula rasa, anything that made him unique as an individual, both good and bad, were cauterised out of his soul. There was nothing left in him; an automaton that was only driven by the will to survive, to forestall death for as long as possible, even if it meant letting others met their end.
The dying screams and pleas for help reached the boy's ears, and he didn't care. The odour of burnt flesh and charred bone reached his nose, and he didn't care. He only knew that he must not stop walking, that if he stopped, death would come quickly.
"I want to live."
It was a wish that resonated with the Curse within the grail. The concept known as Angra Mainyu, the embodiment of All the World's Evil, wanted to be born to exist and fulfil the reason for its conception. Deeper, near the core of the grail, was the slumbering spirit of Avenger, the man whose name was expunged from the Avesta and was forced into the role of Angra Mainyu, also retained the desire to live; though death would have been preferable to most after seeing what he suffered in life. In that instant, the desire of both the manifestation and the basis for the legend were the same. And so the grail granted their wish.
Black mud, curses taken material form, surged out of the black sun tarnishing the sky. It flowed towards the hollow boy, knocking him down as it forcibly entered his body, replacing missing flesh and healing wounds. The sleeping spirit of Avenger, flowing with the tide, was deposited into the boy, like water filling a hollow jug.
If this process continued, Servant Avenger of the Third Heaven's Feel ritual would have walked the earth once more. As it was, the Magus Killer found the boy first, dragging him from that black pool of swarming curses. Without hesitation, Kiritsugu Emiya removed Avalon from his body and implanted into the boy, despite knowing that the holy sheath was all that kept the curse of the grail at bay.
A sheath crafted by the faeries, extensions of the World itself, Avalon acted as a gate to the Pure Land, fixing any physical damage and purging any spiritual taint. The curses of Angra Mainyu were burnt away, and the spirit of Avenger screamed, for it was corruption incarnate, the antithesis of what the sheath represented. But with Saber dispelled, the prana required for Avalon to function was limited. Though he was the weakest of Servants, Angra Mainyu was still a Heroic Spirit, so he managed to survive the attack but was prevented from overriding the boy's soul, and so was forced into a state of hibernation.
And so, a sword was forged.