The Facts in the Case of M. Valdemar is about a mesmerist, who puts a man in a suspended hypnotic state at the moment of death. An example of a tale of suspense and horror, it is also, to a certain degree, a hoax as it was published without claiming to be fictional, and many at the time of publication (1845) took it to be a factual account. Poe toyed with this for a while before admitting it was a work of pure fiction.
(Excerpt from Wikipedia)