This is not a core rulebook. There are no new systems players must memorize before play. The mechanics here exist to support planar travel, creatures, relics, and DN storytelling. Every world that has ever been run at an Open Dungeons table exists somewhere in the Multirealms. Every dungeon your players have crawled through, every kingdom they have saved or doomed, every tavern argument they have started - all of it is part of something infinitely larger, and that larger thing has shape, logic, and weight.
The Realms Guide does several things. It describes the cosmological structure of the Multirealms - the infinite oblong of existence that contains all realities, all gods, all hells, all heavens, and everything in between. It explains how the pieces fit together, why magic works the way it does, and what happens to souls when their bodies stop using them. It gives you creatures native to the cosmos itself - things that live in the Ethereal membrane between worlds, things that guard the thresholds between realities, things that crawl out of the deepest Lower Realms. Full stat blocks, behaviors, and DN notes.
It gives you named areas - specific places in the Multirealms with character, survival requirements, and the kind of detail that makes a world feel real when players ask questions about it.
It gives you practical travel mechanics - how you actually get from one reality to another, what that crossing does to your body and mind, how different classes handle the journey differently, and what you need to survive the trip.
And it gives you relics - items that carry the fingerprints of the realms they came from.
None of this is required to run a great game. All of it is available when you want to go further. Use this book as scaffolding, not scripture. The Dungeon Narrator is always the final word. DNs make the universe for their players.