Mystara: a world created by TSR to accompany the Dungeons and Dragons boxed sets in the 1980s, and explored in a series of Gazetteers, which are published settings detailing the history, geography, society, locations and characters of the country.
Melestrua's Mystara is my take on this, centred on the "Known World" which forms the eastern end of a large continent in the north of the world of Mystara.
I have been using Mystara for my campaigns since 2017 and have now run several campaigns within the world. As part of each campaign I have fleshed out areas of Mystara in greater detail, and I am now putting it up on World Anvil for wider consumption in case anyone else finds it useful.
To start, see The "Known World"