The Yahirono Himorogi is an important artifact guarded by the fairies at Yoyogi Park. Several people entered the park searching for the stone to use its power for their own goals. Sakahagi stole the stone and inflicted the fairies with a curse that robbed them of their senses. Chiaki entered the park and fought with Sakahagi before the fairies were cursed; she was defeated and maimed by Sakahagi (she lost an arm), yet managed to survive and limp away to safety.
Yuko also entered the park to find the stone, but soon gave up after sensing that she stood no chance of defeating the current possessor. She soon encountered the Demi-Fiend and told him her purpose for wanting the stone; to force Aradia to give her a Reason (not realizing that she is the one who's supposed to conceptualize a Reason, or that Aradia is not even a 'real' deity, excluding her from the list of viable gods). She then asked the Demi-Fiend to retrieve the stone in exchange for the Magatsuhi contained within it.
After managing to get through the pixies' traps, the Demi-Fiend met Sakahagi at Cyber's construction site. Sakahagi told him that he already had used up all the Magatsuhi in the Yahirono Himorogi and that he would be the one using demons for now on - not the other way around. Sakahagi summoned a Girimehkala to fight the Demi-Fiend, but the protagonist defeated both of them. After getting the stone he was thanked by Oberon and Titania for lifting the spell affecting the fairy clan. They allowed him safe passage with the Yahirono Himorogi and bade him adieu.
After leaving the area the Demi-Fiend returned to Yuko and gave her the stone. Aradia possessed Yuko, asking him two important questions that would affect his destiny. After answering her, Aradia tells the Demi-Fiend that a Reason will soon be established in Ikebukuro and leaves. The Harlot would later express deep surprise that the stone was handed over with so little fuss, despite the terrible struggle to acquire it in the first place.
Later, as the last Reason was being established, Yuko was used as a sacrifice to summon Hikawa's demonic sponsor, Ahriman. Before she fades away, Yuko gave the Yahirono Himorogi to the Demi-Fiend and told him that his will was stronger than hers and that he should create his own world by using the stone to build a path to Kagutsuchi.
The Demi-Fiend headed to the Amala Temple and placed the stone on an altar in the central pyramid. The stone activated and sent a beam of Magatsuhi to Kagutsuchi, who lowered his tower on top of the Obelisk, driving it into the ground, connecting itself to the Vortex World.
- Yahiro is a word that means "enormously huge."
- A Himorogi is the most basic form of a Shinto shrine. In Shintoism, everything has a soul in nature and objects that hold very pure spirits are "Himorogis," which are usually covered with special paper. Basically, a "Yahiro-no-Himorogi" is an oxymoron, since it means a small basic object of gigantic size.