The Mysteries of Udolpho (1794) is the archetypal Gothic novel. A young woman, Emily St. Aubert, suffers the death of her father, followed by worsening physical and psychological death, mirrored in a landscape of crumbling castles and emotive Alps.
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Severity reflects intensity, not value -- “central theme” means a warning is a core part of the book, not that the book is bad.