From the introduction: This book is an unofficial, non-commercial fan update for the fifth edition of the worlds [sic] greatest roleplaying game. It takes a number of sets of rules from previous editions of the game and updates them to make them compatible with the fifth edition rules. Most of the rules contained in this book are taken from the boxed sets published in the early- to mid-eighties and commonly known as the “BECMI” sets (from their initials - Basic, Expert, Companion, Master, and Immortal) and it is from those names that the name of this book is derived. User summary: Includes fan conversions of five sets of rules from older editions of D&D to fifth edition: Essence (magic item creation rules adapted from "residuum" in D&D Fourth Edition) Dominions (rulership and mass-combat rules adapted from D&D Companion Rules set) The Multiverse (planar cosmology adapted from D&D Immortals Rules set) Immortals (adapted from D&D Immortals Rules set) Chronomancy (time-travel rules extrapolated from various D&D BECMI adventures)