Short answers, pulled from the story.
Mystara is a campaign setting for the Dungeons & Dragons fantasy role-playing game. It served as the default setting for the Basic version of D&D through the 1980s and 1990s. Most Basic-edition adventures took place on the Known World, its central continent of human and non-human realms.
Lawrence Schick and Tom Moldvay developed the earliest version of the setting for their own Dungeons & Dragons game sessions between 1974 and 1976. Bruce Heard first published the name Mystara itself in 1991, in Dragon magazine.
Mystara uses ascended immortal beings in place of the gods found in most other Dungeons & Dragons settings. These Immortals live on the moon Matera, in a city called Pandius.
The Hollow World is a habitable interior surface inside Mystara, a hollow planet, lit by an eternal red sun. It preserves societies that vanished from the outer world, including cultures based on Native American, Aztec, Viking, and ancient Roman civilizations.
Mystara's third edition was released in 2000. By then, Wizards of the Coast had already discontinued its support, and the fan-run Vaults of Pandius site had become the setting's official home.
Five video games used the Mystara setting across three genres. Order of the Griffon and Warriors of the Eternal Sun, both released in 1992, were role-playing games, followed by the 1993 strategy game Fantasy Empires and the arcade beat 'em ups Tower of Doom and Shadow over Mystara.
The Red Curse is a sinister enchantment on Mystara's Savage Coast that kills inhabitants through mutation. Wearing a fictional metal called cinnabryl against the skin protects against it, though leaving the cursed area brings on debilitating effects of its own.