Wikipedia
Thyatis
The Empire of Thyatis is a powerful state in the Mystara campaign setting for the Dungeons and Dragons fantasy role-playing game. Thyatis was first mentioned in Module X1, Isle of Dread, which describes it briefly. Along with the Alphatian Empire, Thyatis was more fully detailed in the Dawn of the Emperors boxed set. Along with Alphatia, Thyatis is one of the " great powers" of Mystara.