The Devil's Foot

Holmes is resting on the Cornish coast at Mount’s Bay when the “Cornish horror” case occurs there: two brothers go mad, and their sister dies right at the table with a contorted face. Then one more client of Holmes dies as well—also with a face of horror after death. It turns out that the deaths of the two people and the brothers’ madness were caused by an admixture known as “the Root of the Devil’s Foot.” The culprit is Leon Sterndale. It was he who killed Mortimer Trejennis for the death of Brenda, the brothers’ sister. With a powder, Trejennis deprives the two brothers of their minds, and gives the sister death. Sterndale takes revenge by pouring the same powder into a lamp, after which Trejennis dies.

Fact 1: Arthur Conan Doyle included this story among the 12 best Sherlock Holmes stories.
Fact 2: This is one of the stories in which Holmes lets the criminal go free.