The Labyrinth Worldbook is a source book by Kobold Press for Tales of the Valiant or any DnD 5e variant. This is my review of chapter 2: Worlds of the Labyrinth.
Chapter 2 contains information about the major "worlds" of the Labyrinth, including the hub city known as The Smithy. Well, the name of the chapter is Worlds of the Labyrinth but the term "world" isn't entirely accurate. Recall that the Labyrinth isn't a linear maze, like the ones scientists drop mice into, or a hedge maze you might enter on a summer afternoon for a fun diversion. It's only a maze in the sense that it's a complicated and irregular network of pathways that are difficult to navigate. Kobold Press's Labyrinth is a cosmic puzzle box, not a metaphysical maze. There are many paths through it, and many stops along the way.
The Smithy is a city founded by travelers of the Labyrinth who needed a homebase. Specifically, the city rose up around the first connection that got made between the world of Seven Pines (in the Old Ring) and the Great Maze of the minotaurs. It's a major city, with a population 50,000 (not counting migratory merchant and warrior and wanderer populations), and is home to the Citadel of the Keepers and to a great many dimensional portals and other Labyrinth pathways. Its ruler is Haldevar, the Queen of the Labyrinth and servant of the spider goddess Rava.
Most importantly for the storytelling part of your RPG, the Smithy is a location that trades in portal keys. Dimensional portals are cool in a game, but they're only special if only a few people have access. To ensure that the player characters are just those few people, you need a reasonable source of the key that unlocks a portal. The Smithy has 50,000 people in it, which feels like just enough to be a busy location, but a lot less than, say, the combined populations of a near-infinite number of realities.
There's nothing but pathways in and around the Smithy, but a few notable ones are mentioned in the text:
Those are only the most prominent gateways contained within the Smithy. There are many others, including the Fire Wheel, the Lunar Beamway (not the same as the Moonbeam Arch), the Narric Trace, the Spindle, and many others. The point is, the Smithy is written to be a hub of planar travel. Your players, like the Keepers of the Keys themselves, can use the Smithy as a point of reference and a literal gateway to adventure.
Any good magic user knows about Astral Projection. It's when a mage visits a place without actually visiting it. The mage appears to be in the location, but physically the mage hasn't maved to that location. Instead, the mage just travels the Astral plane, which intersects with all realities. The problem with Astral Sea is that it's dangerous, because it's home to monsters and derro corsairs and worse. Even so, the Astral Sea is one method you could use to traverse the Labyrinth.
In some cosmologies, the elements fire, earth, wind, and water emanate from basically single-purpose planes. The plane of fire is infinite fire, and only fire, and when you strike a match or cast a fireball spell, you are essentially summoning the element of fire from its home plane. In the Labyrinth, this is still true, except that the "planes" aren't flat and infinite, but round and (presumably, but also paradoxically) infinite.
The elemental spheres of the Labyrinth are deathtraps to the unprepared traveler, but they have a lot of potential for adventure. The elemental sphere of fire features the City of Brass, ruled by efreeti and frequented by the azers, and salamander clans in the Ash Wastes. The sphere of earth has a council of warring khans, and the djinn in the sphere of air have built the great city of Qasfi Alasam. In the elemental sphere of water is the metropolis of Kinvlemere, which connects to the Dreamer's Islands and the banks of Styx. With a few magic items or the blessings of a god, player characters could travel to these planes and have a lot of outer-world fun.
The Ten Thousand Worlds is a "cluster" of realities that can be assumed to include any other campaign setting you care to use in your adventure. I have the feeling, based on nothing but my own interpretation, that "ten thousand" is intended in an archaic sense, which is to say "innumerable." You can assume that it's home to Pathfinder's Golarion, Dragonlance's Krynn, Tolkien's Middle Earth, D&D's Toril, and so on.
The ether suffuses the Ten Thousand Worlds (ignoring a typo in the book saying "Ten Thousand Words"). It's the non-substance that ghosts move through, or sometimes invades your sleep. Using magic, some mages can enter the ethereal plane, and can even use it to travel from one world to another (including the elemental spheres.)
Based on the world tree of Norse mythology, Yggdrasil's branches form the Oak Road that winds through much of the Labyrinth. It's home to ratatosk (squirrelfolk that consider themselves not beastkin but divine messengers), ravenfolk, and eagles. It's under constant assault by servants of evil Nidhogg, the serpent god that seeks to devour Yggdrasil.
The river that runs through the Labyrinth and, ultimately, to the 11 Hells.
The Labyrinth Worldbook defines 10 prominent worlds with direct ties to the factions and pantheons of the setting. Many many more exist, obviously, and it's a community effort to define them. Here are the ones provided in the book:
I've provided a one-line summary, but the book contains at least a page on each world. It names cities and locations within each world. It also identifies which factions and gods and characters have a vested interest in each world. You can almost certainly find an adventure module or an entire campaign that's set in one of these realms, sometimes explicitly (such as the Old Margreve campaign, or the Courts of the Shadow Fey) and other times implicitly (Out of the Abyss could be situated within the 11 Hells, Tomb of Annihilation or Frog God's Tehuatl could be set in the Southlands, and so on).
Basically the Nothing from Neverending Story, or existential dread from your own real life. The Void is the external threat of non-existence, and the source of void monsters. The Void is ever encroaching upon the Labyrinth, and even the 11 Hells fear it (in fact, fiends from the 11 Hells often have the goal of sacrificing a world to the Void to hold back their own extinction.) There are several pages about the Void, and many void aberrations in Kobold Press bestiaries.
If you're already a fan of Kobold Press, then you may find the worlds of the Labyrinth familiar. It obviously includes existing Kobold Press properties, which means that if you've been using Kobold Press material then you can easily transport your player characters to a world that's familiar and fully developed. On the other hand, if you're excited to explore and discover new places and create new adventures, then the Labyrinth provides a narrative gateway for their discovery.
I don't think the authors of this expansion could have chosen a worse name for it. The Labyrinth is obviously not a labyrinth, or even a singular construct. Its key feature, as far as I can tell, is the idea that while you can technically get to any world from any other world, in the Labyrinth you often can't get there directly. For a direct route from one place to another, you must find the path that leads to it, and even then you probably have to spend time on that path. As written, it seems to be a system to encourage the travel part of inter-dimensional travel. It's more like a network than a maze.
I'm a little disappointed at how lifeless the Smithy is, especially if I'm supposed to use it as the hub for my adventures. It's a bland fantasy city with some big buildings and no notable personalities. It's a setting, but it feels like the bare minimum. It's like something you come up with when you didn't expect your players to discover the multiverse. Van Richten's Guide to Ravenloft offered a lot less than its predecessors, but at least it gave each domain a personality (mainly through its dark lords). Guildmaster's Guide to Ravnica gave its city personality through audacious guilds. The Smithy, sadly, is just a city with 4 factions and a bunch of roads and portals to the tangle that is the multiverse. As a city, it adds nothing, and I'd much rather run the Labyrinth from a city I already know.
Next, I'll review Chapter 3& Factions of the Labyrinth.