Most adventures published for the "Basic" edition of D&D take place in "The Known World", a central continent that includes a varied patchwork of both human and non-human realms.

[2] Mystara originated as a fantasy world developed by Lawrence Schick and Tom Moldvay for their own Dungeons & Dragons game sessions from 1974 to 1976.

Inspired by author H. P. Lovecraft and his work in creating a fiction shared universe, Schick and Moldvay named their setting as the "Known World" so it could be expanded upon by other players.

[6]: 19  The first published use of the name "Mystara" came in 1991 from Bruce Heard in the Letters section of his Voyage of the Princess Ark series in Dragon magazine.

Each part of the D&D Gazetteer series treats one nation or empire and has three basic elements: cultural and geographic background, features, and adventures.

The cultural and geographic campaign background section offers a brief history and timeline for each nation; basic geography, climate, and ecology; and, fundamental social and political concepts of the region.

[10] Nations created for the world of Mystara include the Thyatian Empire, the Grand Duchy of Karameikos, the Principalities of Glantri, the merchant-run Republic of Darokin, the Emirates of Ylaruam, the Dwarven nation of Rockhome, the Elven Kingdom of Alfheim, Halfling lands of the Five Shires,[10] and the chaotic Alphatian Empire.

[11] As the name implies, the "Known World" covers the most notable nations of Mystara, the ones most commonly used in Mystara-based campaigns and featured in fiction.

It includes the Thyatian Empire, which could be compared to the Byzantine Empire; the Grand Duchy of Karameikos (which includes the town of Threshold, the default setting of many classic D&D adventures); the Principalities of Glantri, ruled by wizard-princes; the Ethengar Khanate, a Mongol-like society; the merchant-run Republic of Darokin, which is based loosely on the mercantile states of medieval Italy; the Emirates of Ylaruam; the Heldannic Territories, ruled by an order of religious knights similar to the Teutonic Order devoted to the Immortal Vanya; the Atruaghin Clans, which have an Amerindian feel; the nation of Sind, based on the Mughal Empire; the Northern Reaches Kingdoms of Ostland, Vestland, and the Soderfjord Jarldoms, based on Scandinavia at various periods of history; the Dwarven nation of Rockhome; the elven Kingdom of Alfheim; the Halfling lands of the Five Shires;[10] and the Alphatian Empire, ruled by wizards and other spellcasters.

[11] To the distant Northwest of the "Known World", across the Great Waste, lies the mysterious lands of Hule, ruled by Hosadus, also known as "The Master".

The Adri Varma lies between Sind, Wendar, the Great Waste, and The Black Mountains, forming the northern border of Glantri and defining the northwestern limits of the region.

Mystara includes the Savage Coast, a coastal area located in the south-central part of the Brun continent, to the south and west of Hule.

This sinister enchantment eventually kills its inhabitants through mutation unless the (fictional) metal cinnabryl is worn in contact with the body.

The specifics of the "Red Curse" include mutilation of the body and extreme degeneration of physical and mental health.

In 1996, the setting was revised and re-released under the AD&D: Odyssey line as three fully online products available for free download.

The only major appearance of the continent was in Dragon magazine, where parts of it were sketched out during the Voyage of the Princess Ark series, by Known World Product Manager Bruce Heard.

The poles are actually huge, subtly curving holes that allow passage between the outer and inner world, although it is a long, hard trek through a cold, unlit, stormy and anti-magic area.