The Red Widow Murders

They say that Lord Mantling's mansion is haunted - at least, one room of it is. Known as the Red Widow's Chamber, the now-sealed quarters once housed the wife of a guillotine operator in the French Revolution, and, since her passing, have been host to a century of unsolved horrors, including the death of a man in 1802, the death of a child in 1895 and a number of mysterious mortalities in the years in between.
Now, in 1935, eight men and women join at the manor for a sinister experiment to determine the truth behind the haunting once and for all: they each draw a card, and whoever pulls the Ace of Spades must spend a night in that terrifying room. But the challenge turns fatal when the man selected for the task is found poisoned the next morning when the doors are opened. The locked room was guarded all night, so nobody could have entered or escaped; what's more, the deadly toxin could only have entered through a break in the skin, but no wounds were discovered on the body.
Is this evidence, at last, of a nefarious spirit at work, or of a diabolical and ingenious killer? Only Sir Henry Merrivale, called in to take note of the night's proceedings, will be able to examine the clues and deduce the truth.