noun, na-adjective
Literary word for deep, quiet darkness or a secluded, shadowy place. Often carries a poetic or classical tone. Rare in everyday speech; more common synonyms are 暗闇 (くらやみ) or 闇 (やみ).
森の奥深く、幽暗な場所に古い祠がひっそりと立っていた。
Deep in the forest, in a gloomy place, an old shrine stood quietly.
彼は幽暗の世界に引きこもり、誰とも会わなくなった。
He withdrew into a world of seclusion and stopped seeing anyone.
Compound of 幽 (ゆう, 'dim, secluded') and 暗 (あん, 'darkness'). The exact historical derivation is uncertain, but the combination conveys a sense of deep, quiet darkness.