On the Central Continent stands an ancient sanctuary, its origins lost beyond telling, like stone bones buried under eras. It has existed since the world first drew breath.

It’s called the Daynight Sanctuary. Inside, nothing but two books rest upon a silence as still as a lake. Yet those two volumes are among the world’s most precious stars.

They are the Book of Day and the Book of Night—complete Rule Books, like the sun and moon etched into law. They hold the laws of light and darkness, tides that govern everything. They are the only Rule Books in existence, rare as twin phoenixes.

For countless years, no one earned their recognition, so no one could wield them, like blades sealed in ice. The Rule Books drifted from memory like mist dissolving at dawn.

Until this generation of the Mizumi Clan, when two girls appeared like paired lanterns in dusk. They are the third and fourth daughters of the clan head, Yugami Yuu—Yugami Rexiao and Yugami Rexi, twins with a light-spirited body and a dark-spirited body, like day and shadow twined.

A few years ago, jealousy pricked like frost when their beloved big brother, Yumigawa Sumeragi, kept close to Yumigawa Nozomi and left them in the cold. Hurt flared first; they ran away like wind-swept leaves. By accident they stumbled into the Daynight Sanctuary, and fate pressed them to accept the legacy of the two books, like crowns lowered onto unwilling brows.

When Yugami Yuu learned of it, worry didn’t cloud him; joy warmed his chest like a hearth. Even so, the short separation tugged at him like a taut thread.

...

On this day, at the Daynight Sanctuary, the heavy doors that hadn’t moved for ages groaned open like old thunder. Two girls, fourteen or fifteen, stepped out through drifting motes like stars.

They mirrored each other in face, expression, build, and height, twin ponds reflecting one sky. Only hair and clothing parted them like dawn and dusk.

The girl on the left had long golden hair, pale as fresh snow, with small braids tied at each side like twin rays. Her clear eyes were blue-green, shining with a gentle glow like lake light. She wore a pure white princess dress, and a white book stamped with a sun hung at her waist like a small rising dawn. From afar she looked like a holy angel, a beam in winter air.

The girl on the right wore black hair more beautiful than night, styled in twin tails that flowed like silk streams. A small ornate hat perched on her head like a crescent crown. Her pitch-black eyes were so deep they felt like wells drawing the soul, starless and calm. Her dress was a black-and-white Gothic lolita style, with black dominant like midnight. At her waist hung a book too, but hers was black, its cover marked with a moon, a silver quiet.

No need to say it—these were the twin sisters who inherited the two books, like sun and moon walking. The elder, Yugami Rexiao, bore the Book of Day. The younger, Yugami Rexi, carried the Book of Night.

She lifted her eyes to the cerulean sky, a soft ache rising like a tide. “Rexi, how many years have we stayed in the Daynight Sanctuary?”

Rexi’s voice was flat, smooth as still water. “Around three or four years, Rexiao.”

A wistful smile touched Rexiao, thin as sunlight in fog. “It’s been so long. Do you think our dear big brother has already forgotten us?”

Rexi’s tone firmed a little, like a stake set in earth. “He won’t, Rexiao.”

Curiosity brushed Rexiao’s heart like a feather. “Why?”

Rexi’s answer was simple and steady, like a mountain under snow. “Because he’s our big brother, Rexiao.”

A laugh escaped, warm as spring. “You’re right. Let’s head back and give our dear big brother a proper scare.”

Agreement settled between them like falling leaves. “Alright. No objections, Rexiao.”

They vanished in an instant with a soft whoosh, like swallows cutting through air, leaving only a gentle breeze to pass by.

...