In 1940, the Fantom strip was written by George Kapitan as well as Paul Gustavson and Frank Thomas. No extant evidence exists that is known as to who wrote this particular story.
Same applies to the artist of the story. Besides Gustavson, other known artists include Al Anders and Will Roland.
The story is clearly dervied from the 1920 German expressionist film 'The Cabinet of Dr. Caligari', with Agar as the Cesare of the movie.
Wolverton signs in the final panel.
Loomis was previously credited with the script, but Bails' Who's Who only gives him as penciller and inker at Centaur.