When the 5e Dungeon Master's Guide DMG came out back in 2014, I read it from cover to cover. I've decided to re-read the 5e DMG, though, to re-discover anything I impatiently overlooked on my first read-through, and I'm going to review it chapter by chapter. Chapter 2 is titled "Creating a multiverse."
Did I say this chapter was called "Creating a multiverse"? What I meant was that it ought to be called "A Planescape Primer," and I mean that in the very best of ways. I'm a longtime Planescape fan, I think partly because among the many landmarks I've experienced in my long strange journey to playing D&D, it was a Planescape book that made me understand that this D&D thing was an actual game and not just a collection of Dragonlance and Forgotten Realms novels. So when chapter 2 casually mentioned the Infinite Staircase, my heart skipped a beat as I had sudden flashbacks of temples of Selûne, Glabrezu, the mysterious Shavanistra, and so many more scenes of planes-hopping.
This chapter, of course, doesn't attempt to get anywhere near the original Planescape boxed set's coverage of the multitude of planes out there. It ultimately boils down to this, as simple as it may seem: There is no physical model for where planes exist, and all cosmologies are merely the creation of mortals in a feeble attempt to conceptualise the ineffable.
I feel like this chapter could do more to explicitly explain the relationship between planes and magic. It mentions Plane Shift and Astral Projection specifically a few times, but it's a complex subject that can generate a lot of confusion during a game, and I wish there were a single place in a 5e book (or any edition, for that matter) that spelt it out. Does Dimension Door cause you to cross into another plane? Does any spell that causes you to teleport cause you to travel through the Astral Plane, or do some use different methods?
Ultimately, questions about how extradimensional spaces and planes and plane shifts function is up to the Dungeon Master's understanding of fantasy quantum physics, and probably the simpler that understanding the better. That usually works well enough. But it does get tricky, especially when you start interacting with planes in the heat of battle or the tension of a dangerous dungeon crawl, and I think it would be nice if there was a section in the DMG to lay it all out in clear terms.
For me, one of the most fascinating things about planar travel has always been that certain planes impose a slightly different version of reality on players. In Planescape, going to a different plane often meant that you experienced a bonus or penalty to certain skills or magical abilities, based on the plane's alignment or general nature. The same goes for traveling to another setting, too. When a cleric from the Forgotten Realms visits Krynn, for instance, they lose their divine powers unless they find a surrogate god.
My experience with these has always been fun, even though they can be difficult to keep in mind. And I admit that as a player I'd probably get annoyed eventually if the cool feature I'd built my character around kept getting nullified because an adventure happened to take place on a different plane.
It's nice that the DMG includes altered rules and effects for a few of the planes, and it's even nicer that the rules are declared optional, presumably in recognition of how it could get frustrating for certain character builds. Obviously all rules are optional, so it's a little funny to declare planar features "optional," but somehow the recognition that a rule is optional makes you feel less guilty when you either ignore or forget it during actual play.
This 28 page chapter isn't a Planescape boxed set, nor a Manual of the planes or Plane-hopper's Handbook, but it is a quick and effective introduction to the planes. What's more, it does an amazing job of unifying the different interpretations of the multiverse that D&D has supported over the past few decades. I don't know whether a Dungeon Master would necessarily be ready to run a custom inter-planar adventure just after reading this chapter, but I do think you'd be ready to run at least a published inter-planar module. This was a fun chapter, and I think it does a great job at establishing a good metaphysical foundation for an inherently magical and mystical game world.
The next chapter is Creating adventures.