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evil spirit; (vengeful) ghost; specter; spectre
病は物の怪の仕業とされた。
The illness was attributed to a vengeful spirit.
廃れた社には物の怪が住み着くという。
It is said that a vengeful spirit dwells in the abandoned shrine.
古い山には物の怪が潜むような冷気を感じた。
I felt a chill, as if a mononoke was lurking in the ancient mountains.