The Council of Eight Immortals uses yāoguài as a working term throughout Jackie. When Lü Dongbin draws the diagram of the modern threat for Jackie in the dining hall, he does not say artificial intelligence or algorithm. He says yāoguài, and the older immortals nod the way grandparents nod when they have seen the thing before. The naming matters: it locates Halo and the methodology not as a novel evil but as the latest member of a long lineage. Fox-women have always whispered the right thing. Hungry-ghost objects have always answered before the question. The lineage is what makes Jackie believable to himself — if a thirteen-year-old must fight a yāoguài, that is a job for which there are precedents.
The category also organizes the book's bestiary. Niu Mowang is a yāoguài. The Xiangliu nine-headed snake is a yāoguài. The unnamed shadow Jackie keeps half-seeing — in the rearview mirror in Chapter 4, on the National Mall in Chapter 14, finally face-on in Brent's office in Chapter 22 — is a yāoguài of a particular kind: one whose form is absence. The books are careful not to confuse all yāoguài with the pitch-black thing; they are siblings in the same taxonomy, not the same individual.
The yāoguài lineage runs through nearly every major strand of Chinese narrative literature, from the Six Dynasties zhiguai (志怪 — "records of the strange") tales of the 4th–6th centuries, through Tang chuanqi romances, Song-Yuan vernacular fiction, the great Ming-Qing novels (Journey to the West, Investiture of the Gods), and Pu Songling's Strange Tales from a Chinese Studio (1740). The thread is consistent: yāoguài are beings whose ontology is suspect — a fox that learned to be a woman, a fan that learned to be a person, a shadow that learned to want.
Critically, the tradition does not treat yāoguài as purely evil. Many are tragic, many are seducible by Buddhist or Daoist conversion, some become protectors. The category is moral-agnostic; it names the wrongness of the form, not the wrongness of the heart. Jackie inherits this nuance — Halo is yāoguài-shaped, but the people who built it are something else, and the books refuse to flatten the difference.
A form that shouldn't be. Yāoguài is defined first by ontological wrongness — something walking around in a shape it has no right to.
The lineage of <em>Halo</em>. The Council names the methodology as yāoguài because it locates a contemporary threat inside a 1500-year-old taxonomy — placing the boy on familiar mythic ground.
Sibling to the void. The pitch-black thing is yāoguài of the negative-space variety; Niu Mowang is yāoguài of the kin-betrayal variety; Xiangliu is yāoguài of the multiplied-head variety. The category contains them all.
Convertible, sometimes. The tradition's moral-agnostic stance is what allows Jackie's ending to leave doors open — yāoguài can be defeated; some can also be turned.