"Unholy Trinity"  
Amazing 198507.jpg
Author Harry Turtledove
First Appearance Amazing
Collected Agent of Byzantium
Series Agent of Byzantium
Publication date July, 1985
Preceded by "Pillar of Cloud, Pillar of Fire"
Followed by "Archetypes"

"Unholy Trinity" (also known as "Etos Kosmou 6824") is a short story by Harry Turtledove (as Eric Iverson), first published in Amazing, July, 1985. It is a story set in the Agent of Byzantium series, and was reprinted in the fix-up novel of the same name in 1987.

The story takes place in 1315, in the Pyrenees and eastward to the Monastary of St. Gall, along the borders of the Byzantine Empire. The barbarous Franco-Saxons had developed a new, devastating weapon of hellpowder which threatened the Empire to which Basil Argyros was assigned to investigate.

The "Unholy Trinity" of the title refers to the monks associating each of the three components which went into making hellpowder with one of the persons of the Christian Trinity of Father, Son and Holy Ghost. It was a convenient mnemonic device for the monks, but eventually it gave the spying Argyros the hint he needed in order to decipher the monks' secret and take it back to Constantinople.