In "Shoku Nihongi" (Chronicle of Japan Continued), there is a description that 'Shinto purification is held because yokai appears very often in the Imperial Court,' and the word yokai is used in a similar manner, not to refer to some object, but to describe an unnatural phenomenon.
-『続日本紀』に「大祓、宮中にしきりに、妖怪あるためなり」という記述があり、同様になにかの物を指すのではなく、怪奇現象を表す言葉として妖怪を用いている。