Strange Yanese Tales of the Supernatural - Supplement (1)
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According to the main corpus of this work, dreams are extremely vulnerable to the Unclean. The work titled "Miscellany for Discerning the Unclean" contains a record of an entity called the Mountain Bearer. According to hypothetical guesses made by later commentators, extremely exhausted individuals may dream about strange sights and experiences. Such dreams might be projections of the Unclean.
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A story recorded in "Inquiries for the Hall of Lotus Silk" describes a school child who saw his grandfather at the other end of an alley waving at him. However, a small critter held on to the boy's trousers by its mouth and prevented the reunion. The boy then woke up and realized it was just a dream.
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It should be safe to assume that the Unclean is born of the heart and is nourished by dreams.