DA1 "Adventures in Blackmoor" includes a section describing "The Empire & The North"
The Great Thonian Empire, as it is known throughout most of the world, is technically a republic (the Third Republic of Thonia). In theory, it is ruled by a senate that periodically selects an executive body of one to six members, called an “Imperial". In practice, an Imperial seldom consists of more than two men - and it is usually but one person, who has come to be known as the Emperor of Thonia.
From DA1 "Adventures in Blackmoor" (1986)
"Empire of Thonia: The Eastern Marshes" (2013) by Havard Fanes et al. Cover illustration by Karl Friedrich Schinkel
Havard Fanes has compiled and edited the first three of a planned series of ten fan gazetteers detailing the ten provinces of the Thonian Empire:
CBI 1 "The Eastern Marshes" (2013)*
CBI 2 "The Thonian Rand: Edge of the Empire" (2019)**
**see also the Fall of the Dwarves (2019) campaign extension
CBI 3 "Province of Bleakwood" (2020)***
***see also "The First Fantasy Campaign: Bleakwood"