The Fuyōheki (不妖璧, "Demon-sealing stone") (nulling stone) is a mystical stone capable of masking yōki, otherwise known as demon energy. It was originally in the possession of the demon-mountain Gakusanjin. Naraku stole the Fuyōheki from Gakusanjin in order to mask the demonic aura of the infant, who is his heart. Inuyasha and his group came across Gakusanjin, who was angry and uncooperative at first, but eventually came to see that Inuyasha was not conspiring with Naraku and was trying to help him.
During his encounter with Inuyasha's group, Gakusanjin revealed that the Fuyōheki dissipated demon auras, and allowed him to remain in a slumber. Gakusanjin never completely trusted them, but gave them crystals that contained his own demonic aura. He told them that the crystals were green when surrounded by their demonic aura, but would look like pure white crystals in the presence of the Fuyōheki.
Naraku first used the Fuyōheki for the purpose of attacking Kikyō by surprise, and he was almost successful in killing her because she did not sense his demonic presence approaching her. The priestess was saved by Inuyasha, and so Naraku fled the battle. Afterwards, Naraku gives the Fuyōheki to the infant, using it to hide the location of his heart.
The Fuyōheki was later absorbed (along with the infant himself) by Mōryōmaru. When Naraku absorbs Mōryōmaru and the infant, Miroku tries to destroy Naraku with his Kazaana in a desperate attempt, but is unsuccessful. He is however able to suck the Fuyōheki into the Kazaana, putting an end to the nulling stone.