noun
fallen angel; Lucifer
Primarily a Christian term referring to angels cast out of heaven, especially Lucifer. Also used in fantasy and anime contexts.
堕天使ルシファーは、かつて最も美しい天使だったと言われる。
The fallen angel Lucifer is said to have once been the most beautiful angel.
このゲームのボスは、闇に堕ちた堕天
The boss in this game is a fallen angel who fell into darkness.
悪魔 is a broader term for 'demon' or 'devil', not necessarily a fallen angel.
Compound of 堕 (だ, 'fall, degenerate') and 天使 (てんし, 'angel'). The term is a direct translation of the Christian concept of 'fallen angel'.