"The Treader of the Dust" is a short story by Clark Ashton Smith. It was published in August 1935 in the Magazine of Fantasy and Science Fiction.
Synopsis[]
Antiquarian John Sebastian obtains the Testament of Carnamagos, a tome of forbidden knowledge, from a book seller and in the process is exposed to the horror and hazard of Quachil Uttaus.
Returning to his house, Sebastian is captivated by the Testament and held in place until Quachil Uttaus slowly advances towards him across a gulf of time and space, ultimately- turning the man to dust.