DA1: The Empire of Thonia

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)

The Thonian Empire is based on The Great Kingdom of the Castle and Crusade Society, which likewise served as the basis for the Great Kingdom of Aerdy in the World of Greyhawk.

"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)*

*see also "The First Fantasy Campaign: Richard Snider's Additions" 

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"

The free pdfs are hosted on The Comeback Inn website, and are highly regarded for their incorporation of primary sources (for more on Havard's vision of Thonia, see this thread).