One rule organizes more of Japan's folklore than any other: the otherworld will live with you only unseen in its truth. Traced from the foundation myth to its modern inversions:
The founding violation. Izanagi lights the comb-tooth in Yomi and sees the rot; the first marriage ends in the first curse, and death's arithmetic begins. The pattern is set at the bottom of the cosmology: looking is the breach.
The dynastic repeat. Toyotama-hime asks one thing at the birth-hut; her husband watches; the sea-princess leaves — and the child of that broken promise grandfathers the first emperor. Japan's own founding line runs through a violated don't-look.
The extreme pole — the look that bills your descendants. Everywhere else the tariff falls on the looker: a marriage ends, a year of life goes, the being leaves. The yato-no-kami of the 8th-century Hitachi Fudoki set the ceiling and nobody has since matched it — witnessing the horned marsh-serpents "would destroy one's entire household lineage." Not you. Your line. It is worth noting where in the record that sits: the harshest look-penalty in Japanese folklore is also among the oldest attested, from the century when settlers were still clearing the marshes the snakes lived in. The law arrives at full severity and softens afterward.
The domestic form — the spouse tales. The crane wife's weaving room (seen, she must go — and the seeing is also an audit: her cloth was her own feathers); the fox-wives from the Nihon Ryōiki through Kuzunoha (exposed, they depart — Kuzunoha alone leaving a forwarding address); the yuki-onna (not seen but told — the taboo extended to speech); the tsurara-onna (undone not by sight but by warmth — every variant of intimacy can be the breach).
The predatory form. Some beings are the trap of looking: the mikoshi-nyūdō grows with your upward gaze (look down, say "seen through!"); ōnyūdō variants fell those they stare at; the jorōgumo's falls-warning is a told-secret trap like the snow-woman's; the nure-onna uses your glance at the baby. The basan is the gentle control case: look, and it merely leaves.
The inversions — where the law flexes. Kiyomori out-stares his massed dead and they melt: at the summit of power (or shamelessness) the gaze reverses. The yamachichi inverts it into mercy: its breath-theft kills only the unwatched — being seen saves; don't sleep alone. And the ushi-no-toki-mairi curse-rite inverts it operationally: the ritualist fears witness — proof the law was always about power, not modesty.
The modern mutation. The urban wing swaps the eye for the mouth: aka-manto and kuchisake-onna punish answering; kunekune alone keeps the classical form pure — look closely and your mind goes. Screens, arguably, made lookers of everyone; the legends moved the taboo to engagement.
What the law means. Every version says the same thing from a different angle: relationship with the other side is possible — prosperous, even loving — on terms of incomplete knowledge. The folklore's deepest claim is that some truths end the arrangement that depended on their concealment. This bestiary's method is the law's one licensed exception: it looks at everything — and labels what the looking costs.