鬼 /o ni/ is a good Japanese equivalent for what you are looking for.
demon blade in Japanese is akuma no tsubasa
In Japanese names, "onise" (鬼子) typically refers to "demon child" or "ogre child." The term "oni" means "demon" or "ogre," while "ko" (子) means "child." It can be used metaphorically to describe a person with a fierce or strong character, often found in folklore and storytelling.
ma o
鬼ランプ Oni ranpu
ダークデーモン
ma o
Hi no akuma
aoi akuma
Furasshu no akuma
'Beautiful demon' would translate to 美しい鬼 (utsukushii oni) in Japanese.
"Boku wa oni desu" means "I am a demon" in Japanese. "Boku" is a casual way to say "I" or "me," while "oni" translates to "demon" or "ogre." The verb "desu" is used for emphasis, similar to saying "am" in English.
youkai, onii, bake-mono