Short answers, pulled from the story.
The name Mystara first appeared in official publications in 1991 when Bruce Heard used it in a letter column in Dragon magazine. The world had been shaping the adventures of thousands of gamers for nearly two decades prior to that moment.
The Hollow World is a habitable inner surface that exists in parallel to the outer world of Mystara. This inner realm is lit by an eternal red sun at the planet's core and serves as a cultural museum preserving societies that have become extinct on the outer surface.
Mystara features ascended immortal beings who were once mortal adventurers rather than a traditional pantheon of gods. These Immortals live in a city called Pandius on the moon Matera where they can meet and watch over Mystara.
The Red Curse is a sinister enchantment that eventually kills inhabitants of the Savage Coast through mutation unless they wear the fictional metal cinnabryl in contact with their body. This curse causes mutilation of the body and extreme degeneration of physical and mental health while imprisoning the region's inhabitants.
Mystara incorporated the Blackmoor setting by placing it in the world's distant past where Blackmoor evolved from a feudal kingdom into a highly advanced civilization. This civilization ended itself in an apocalyptic explosion that changed the climate and geography of the entire planet to create the foundation for Mystara.
The planet Mystara has two moons named Matera and Patera. Matera is a visible moon whose phases govern lycanthropy and houses the Immortals in the city of Pandius, while Patera is an invisible moon known as Myoshima to its inhabitants who have a culture similar to that of medieval Japan.