Mythrules was published in only about 100 copies, distributed at a convention ca. 1978. It was designed as a variant of Dungeons & Dragons, though makes no mention of that game, and eventually was considered wholly original by the authors. It cites The Arduin Grimoire 1: The Arduin Grimoire as an influence and presents an original system for spells and magic partially derived from the Earthsea stories of Ursula K. LeGuin".
The rulebook is divided roughly into three parts of equal length - character creation, spellcasting and magic, and combat.