World of Tiers

Farmer's sci-fi/fantasy series follows immortal, human-like "Lords" who construct their own artificial, customized pocket universes and battle each other for dominance. Zelazny took this conceptual framework to create the multiverse of Shadow. Amber and the Courts of Chaos are the two fixed realities, while everything in between consists of infinite, parallel worlds. The royal siblings act exactly like Farmer's squabbling, god-like Lords—possessing the unique power to manipulate, traverse, and rule over these lesser realities.

Gary Gygax lists it in Appendix N of the 1979 Advanced Dungeons & Dragons Dungeon Master's Guide, his published list of the fiction that shaped the game. Philip José Farmer is listed for "The World of the Tiers series". Its pocket universes, made by godlike Lords and linked by gates, are one of the sources for the game's idea of separate planes reached through portals.