WotI’s first book (“Codex of the Immortals”) replaces the Immortal Rules (1986), updating the way Immortals are treated in the D&D game (their powers, hierarchy, interaction with mortals, etc.) and includes a section of Artifacts, as well as some new monsters. The second booklet, The “Immortal’s Fury”, is a campaign saga which redefines the World of Mystara over the years AC1004 to AC 1009.

This wasn’t a simple replacement of course. TSR – and to be fair, other game companies as well – liked to inflict upheavals in campaign settings with the introduction of a new edition. Mystara, D&D’s original “Known World” campaign setting, got the same rough treatment in Wrath of the Immortals.