Zathrum: d20 RPG Solo Engine

From the introduction: Zanthrum is a Game Master Emulator or Solo Engine that allows you to play any Tabletop RPG in solitaire mode, with no need of a GM and other players. This system uses only one twenty sided dice (1d20 for short) and is compatible with every tabletop RPG. ... If you are curious about the name, Zathrum means 20 in Etruscan, an ancient and almost lost language.