Double Ennie Nominee!
The Dictionary of Mu is (usually) a physical book, a pulp fantasy setting supplement for Ron Edwards' Sorcerer RPG where demons are dead things, bits of history made wrong. Players will decide the fate of the planet through play, adding to the dictionary as they go and creating a deeper and richer history with every demonic summoning.
This is a companion discussion topic for the original entry at https://rpgtrader.com/products/1411/the-dictionary-of-mu