For Andrew Huff, lover of horror films, the 75 steps in Washington, D.C., where Father Karras plummets to his death in “The Exorcist” are his Lincoln Memorial. “I go to the steps all the time,” he said, “and when visitors come to Washington, I always take them there.”

All that was missing was a special tourist designation. And on Friday, largely through Mr. Huff’s efforts, that oversight will be rectified. The eerie stairway will be commemorated with an official city plaque — even signed by Mayor Muriel Bowser — declaring them “The Exorcist Steps.”