This is the first Doctor Strange story. It's clear there's a lot of history between Strange and Nightmare. Doctor Strange resembles Vincent Price, who played a magician in the Roger Corman-Edgar Allan Poe film, The Raven (1963).
Inker is obviously George Roussos, though not credited on artwork.
This is the first mention of Dormammu. The Ancient One's home is said here to be in India, this is later changed to Tibet.