Known World (Mystara) Pre-History

Apparently the classic Dungeons & Dragons "Known World" setting -- later "Mystara" -- originated as a home-brew campaign world created by Lawrence Schick and Tom Moldvay in 1974-76.  Unsurprisingly (to anyone familiar with the D&D module X2), Clark Ashton Smith was a major inspiration.

Schick has the details in this post at the Black Gate.