Worlds & Realms draws on, collects, and presents the various planes and noteworthy realms and cities within, all from the point of view of Mordenkainen, a well-traveled wizard. In this way, it's almost like reading a Frodo's Travel Guide of the multiverse. However, it's not a completely impartial view, but flavored with Mordenkainen's belief in the importance of maintaining "the Balance" across the multiverse, as he's come to call it. The idea is that it's not just merely a battle of Good versus Evil or Order versus Chaos that needs to be won by one side; the Balance depends on the existence of each of these... none must overcome and do away with the others.
So, what planes and realms are covered within the expansive confines of this tome? I'm so glad you asked. The book's chapters are divided into three parts. The first part, The Material Plane talks of Greyhawk, where Mordenkainen was born and spent his childhood; there's the hidden world of Mystara; Krynn, the world that serves as the setting for Dragonlance; Toril, the world in which one will find the illustrious Faerûn and The Sword Coast; and war-torn Ebberon, where a horrible war was waged by artifices known as Warforged... until they suddenly and inexplicably gained sentience and went off to live more pleasant lives. This, of course, only touches the surface, as Mordy speaks at length on these places, touching on Oerth, Tharizdun, Vecna, Tasha*** and Acererak and his fabled Tomb of Horrors. And that's just in Greyhawk. He expounds upon the noteworthy tidbits to be shared for each of the contained realms, pointing out the unique features of each of these settings and at least hinting at the great adventures to be found within. If one seeks adventure anywhere in the multiverse, it's quite wise to seek guidance from someone with experience and Mordenkainen is the definitive sage to seek for just such information.
In the second part, Inner and Parallel Planes he shares accounts of the Feywild, the magical realm of fairies and extreme emotions, where the mundane is rare and odd; and soul-crushing Shadowfell, a realm of gothic horror, where one might find Ravenloft and Count Strahd von Zarovich... if they're quite, quite unlucky.
The third and final part, Beyond the Material Plane covers Spelljammers - a magical way of piloting a ship between worlds - as well as various planes that souls travel to naturally in the afterlife, including the Nine Hells, the Abyss and Sigil, a realm between realms, where everything is connected and peace is kept by an entity of incomprehensible power named the Lady of Pain. She actually only steps in when needed, according to laws that are known only to her, but seeks not Good or Evil, Law or Chaos, but Balance within Sigil. Additionally, this chapter touches on the Outer Planes, which consist of various post-life destinations. This chapter also includes the Far Realm, which is the most distant realm from the Material Plane and the most primitive and aberrant, corrupting the other planes where it touches them - which should be nowhere, but in the aberrant nature of the Far Realm, it somehow, inexplicably, happens and when it does, can provide serious danger to the maintenance of the Balance.
In addition to voluminous and verbose accounts of Mordenkainen's personal encounters, observations, and investigations into these worlds and realms, there are also twelve two-page-long short stories, each set in one of the covered realms. These are written by various contributing authors and give a glimpse into the setting, with at least one possible look and feel of the adventures to be had there.
Oh, and the artwork! If a picture's worth a thousand words, then the text in this tome is trying to keep things even, but even so, the artwork within is beautiful and captivating, ranging from small images taking around a quarter of a page to full-spread two-page expanses that invite the reader to get lost for a while in the depicted scene, scrying, perhaps, upon some otherworldly realm.
If you're looking for a D&D source book with stats and such, keep adventuring. However, if you're looking to set your adventures for sights unseen and to explore beyond the Material Plane, perhaps for higher purpose - such as taking up the banner of Balance - or perhaps to seek some treasure or rescue someone known to be stranded in some other realm, on some other world in some other plane of existence - then this is the tome for which you seek. The quality is impeccable, the knowledge is invaluable, and the art is immersive. If you seek a guide to the various Worlds and Realms within D&D, the account of the magnificent Mordenkainen awaits.
* Perhaps I should...
** Well, as him and from his point of view. I have it on good authority that Adam Lee might have had a hand in it, as well as contributions from Jasmine Bhullar, Geoffrey Golden, Jody Houser, Eric Campbell and Jaleigh Johnson. Additionally, there are three pages of small print listing out the artist credits, which are too numerous to list, here.
*** Tasha, also known as Iggwilv the Witch Queen, adopted daughter and apprentice of Baba Yaga, mother of Iuz and, eventually, friend - or at least acquaintence - of Mordenkainen. (See Tasha's Cauldron of Everything review - link below.)
