"The Terrible Old Man" is a short story by H. P. Lovecraft, written on January 28, 1920, and first published in the Tryout, an amateur press publication, in July 1921. It's notable as the first story to make use of Lovecraft's imaginary New England setting, introducing the fictional town of Kingsport.
The Terrible Old Man is a strange elderly man "so old that no one can remember when he was young, and so taciturn that few know his real name". He lives alone in an ancient house on Water Street in the town of Kingsport.
The Terrible Old Man reappears in the story "The Strange High House in the Mist" in a more benevolent role.