Dawn of the Emperors is a campaign setting in the world of the GAZ series, and describes the two warring empires of Thyatis and Alphatia.
The 32-page "Player's Guide to Thyatis" describes an expanding, eclectic empire reminiscent of the Roman that reserves its highest regard for warriors; this book includes character creation rules for Thyatian player characters.
The 32-page "Player's Guide to Alphatia" describes a chaotic, bizarre, and ancient empire where magic-users reign over all non-magical folk; this book includes rules for Alphatian character creation, and for making magic items.
The 128-page "The Dungeon Master's Sourcebook" includes a history and atlas for each of the two featured empires, plus details of disputed territories, campaign and adventure scenario suggestions, and rules for conversion to AD&D.
Dawn of the Emperors - Thyatis and Alphatia was written by Aaron Allston, with a cover by Clyde Caldwell and interior illustrations by Stephen Fabian, and was published by TSR in 1989 as a boxed set containing a 128-page book, two 32-page books, and two large color maps.