Thyatis (the city) is the capital of the Empire and the largest city on Mystara. It is on the southeastern coast of the continent of Brun, situated between the trade lanes of the Sea of Dread to the south and the Sea of Dawn to the east, alongside a canal ("Vanya's Girdle").

Conceptually, Thyatis is a mixture of different parts of the various historical Roman empires. It is governed by an Emperor and an Imperial Senate as per the classical Roman Empire, but enfranchises a feudal noble class similar to that of the Holy Roman Empire. The Emperor himself is akin to that of the Byzantine Empire (as is the capital city's placement on a major strait). The Imperial Bureaucracy as described in Dawn of the Emperors is clearly inspired by that of the British Empire, as is, at least in name, one branch of the Thyatian Armed Forces, the "Retebius Air Corps" (RAF), which consists of cavalry on flying mounts (Pegasus, Hippogriffs, Griffons, Sphinxs, Rocs, and Dragons).

The Grand Duchy of Karameikos is ruled by a Thyatian nobility, but the Empire's territories consist of the western half of the Isle of Dawn, the island realms of Ochalea and the Pearl Islands, and the Thyatian Hinterlands on the southern continent of Davania, in addition to its mainland territories on Brun, which are the smallest of the Empire's territories by geographic size, but the most heavily populated and developed of them. The total land area of the Empire is approximately 800,000 square miles. Because of its political geography, Thyatis can best be described as a thalassocracy.

In its long history, Thyatis has ruled other lands, including Traladara (which became The Grand Duchy of Karameikos), the islands of The Kingdom of Ierendi, the southern half of The Emirates of Ylaruam, and the city of Oceansend in The Kingdom of Norwold. Thyatian settlers have colonized part of the The Savage Coast and also the Heldannic Freeholds, which are ruled by a religious order devoted to the Thyatian-descended Immortal Vanya.

Thyatis has a historic rivalry with the Alphatian Empire, and the two imperial powers devote much of their energy and attention to each other. At the start of the official timeline (AC 1000, or thousand years after the crowning of the first Emperor of Thyatis, in the Thyatian calendar), they are locked in a struggle resembling that of the Cold War, and several published high-level modules have their territorial disputes as a backdrop (notably CM 1, M1, M2, and M5).

"Mainland" Thyatis, which includes several islands, has 3,200,000 inhabitants and consists of the following dominions:

• Actius a small southeastern island County known for its shipbuilding and sea elven community.
• Biazzan a trading city in a mountain pass in northwestern Thyatis, home of a respected university.
• Borydos an eastern island used as an imperial prison.
• Buhrohur a mountain Barony inhabited by dwarves, in northern Thyatis.
• Carytion a lush eastern island used as a luxury retreat by Thyatis' wealthy.
• Halathius a mountain County in northern Thyatis renown for its gold mines.
• Hattias a County covering a large island directly south of Thyatis City, across Vanya's Girdle, known for its population of militant, often racially jingoistic, Thyatians.
• Kantrium a small Duchy in eastern Thyatis.
• Kerendas a large, populous Duchy known for its horse breeding and for training the best cavalry in the Empire.
• Lucinius a county on a peninsula east of Thyatis City, known for its shipbuilding and maritime skills.
• Machetos a small Duchy in western Thyatis, bordering The Grand Duchy of Karameikos.
• Mositius an island Duchy in eastern Thyatis, known for tourism and the magical mists that often cover the island.
• Retebius a Duchy in eastern Thyatis renown for training Thyatis' aerial cavalry.
• Sclaras a small island in southeastern Thyatis, carved into 250 160 acre estates, each held by a powerful Thyatian archmage (36th level D&D).
• Tel Akbir eastern Duchy inhabited by Alaysians, known for their light cavalry skills.
• Terentias island in western Thyatis known for their seafairing and swashbuckling ways. High proportion of elven inhabitants.
• Thyatis, the Duchy of - dominion held directly by the Emperor, most densely populated realm in the Empire and a center of war and trade "for a thousand years, and may be for a thousand more" per Dawn of the Emperors
• Thyatis City - largest city in Mystara, situated on Vanya's Girdle and along the Mesonian River, north of the island of Hattias, in the center of the Dutchy of Thyatis. The Empire's "Heart" and Mystara's Rome or Constantinople. Note that the inhabitants don't refer to it as "Thyatis City", they simply call it "Thyatis".
• Vyalia a woodland County in western Thyatis inhabited by the Vyalia Elves. Known as the home of the Foresters, a fighting order of wizard-warriors.

Thyatis' overseas territories, which cover most of the Empire's land area, include:

• Ochalea a large island domain south of Thyatis, west of the Isle of Dawn
• The Pearl Islands an island chain south of the Alatian Isles and west of Belissaria.
• Caerdwicca a domain on the southern coast of the Isle of Dawn.
• Furmenglave a small domain near Caerdwicca on the Isle of Dawn.
• Provincia Septentriona a province on the Shadow Coast of the Isle of Dawn.
• Provincia Meridiona a province on the Shadow Coast, south of Septentriona and north of Furmenglave on the Isle of Dawn.
• West Portage a trading port in the neck of the Isle of Dawn.
• Redstone a fair domain on the western portion of the Isle of Dawn, opposite Thyatis itself across the Western Sea of Dawn.
• Kendach a fortified county in the center of the Isle of Dawn.
• Westrourke a large Grand Duchy in northwestern Isle of Dawn.
• Helskir a sometime dominion at the northern tip of the Isle of Dawn.
• Thyatian Hinterlands a recently conquered dominion on the northern coast of the continent of Davania, south of the Sea of Dread and west of Ochalea.

The adventures of the boxed set Wrath of the Immortals takes place within the context of a great war between the two, climaxing in the weakening of Thyatis and the sinking of the continent of The Alphatian Empire. Subsequently, Thyatis conquered a few formerly Alphatian territories, suffered setbacks and plague, and by the final official publication that made much mention of Thyatis (Joshuan's Almanac) seemed to be on the road for a gradual recovery. Many campaigns did not use Wrath of the Immortals and the development of their versions of Mystaran countries, particularly Thyatis and Alphatia, may have diverged greatly. In particular those who used earlier published modules, such as CM1 Test of the Warlords or M5 Talons of Night, which involved different conflicts and possible resolutions, could have ended up with very different versions of the Empires than what occurred in the Wrath timeline.

• Dawn of the Emperors Thyatis and Alphatia
• DDA 1 Arena of Thyatis
• DDA 2 Legions of Thyatis
• M5 Talons of Night
• Trail Maps
• Wrath of the Immortals
• Poor Wizard's Almanac
• Poor Wizard's Almanac II
• Poor Wizard's Almanac III
• Joshuan's Almanac