na-adjective
Describes a state of cultural or moral decline, often with an aesthetic of refined decay or indulgence. Used as an adjectival noun (na-adjective).
from French décadent
彼の部屋はデカダンな雰囲気に満ちていた。
His room was filled with a decadent atmosphere.
その小説はデカダンな美しさを描いている。
The novel depicts a decadent beauty.