I was half-way through writing a post about 5th Edition Dungeons & Dragons before I realized that so much I want to say about that game requires me to first talk about 13th Age as a point of comparison. If I had to pick one game to play that is in the broad family tree of Dungeons & Dragons, I would pick 13th Age. But, why?
13th Age was designed by Jonathan Tweet and Rob Heinsoo and released in 2013 (a year before 5e, which is a relevant point that I will get to in a bit). Tweet and Heinsoo have described 13th Age as a "love letter" to D&D, and you can clearly see the influences of their previous projects in 13th Age (Tweet was a co-lead on D&D 3rd Editon, Heinsoo on 4th). It is a d20-based game, published under the Open Gaming License, using all of the standard D&D tropes and concepts. Indeed, the core rulebook is written in a manner that pre-supposes that you have played and are already familiar with other D&D derived d20 games. There are several places in the core rulebook where it says "hey, if you want to import this traditional D&D concept [alignment is an example], more power to you," without at all explaining what the concept is or how it works. In their defense, the intro to the corebook describes 13th Age as being a game for experienced GMs (read: experienced d20 GMs), so they are transparent about what they are doing. Still, my primary complaint about 13th Age is that the core rulebook is not well organized and is a bit impenetrable, even for folks who know what they are looking at, as 13th Age changes enough stuff that you have to read carefully to figure out what is going on. The later offerings in the 13th Age line are much, much better organized, perhaps showing the influence of the good folks at Pelgrane Press who publish 13th Age.
In any event, while 13th Age is clearly in the broader d20 family, it is distinct in several ways. Tweet and Heinsoo are writing a love letter to D&D, but more specifically they are writing a love letter to the kind of D&D that they like to play. After all, there have been many different, and even somewhat mutually exclusive, ways to play and approach D&D in its 40+ year history, and 13th Age unambiguously picks a lane. Specifically, 13th Age is designed to play a high-powered, cinematic version of D&D. 13th Age characters start out powerful and grow to become almost demigods, and are deeply enmeshed in the great deeds and goings on of the most powerful figures in the campaign world. In a broader sense, 13th Age is an opinionated game, in the sense that the designers are very clear about what they are trying to do and what they think is cool about D&D, and it oozes from every page. There are frequent side bars where the designers talk directly to the reader/GM and discuss the thinking behind a certain rule or monster. It's direct, but it is not preachy or confining; instead, it comes across more as having a casual conversation with the designers about the game. There is also a clear sense of humor than runs through the text--the entry for the "Demon Toad" in the pre-release copy of the Book of Demons that I have in front of me begins with the following bit of flavor text:
“Cover your mouth when you burp,” say parents near the Hell Marsh, “or the demon toads will hear you and eat us.” Yeah, it’s pretty dark over there.
This is certainly not the only way to approach D&D, and I don't sense that you would get any disagreement on that point from Tweet and Heinsoo. It is completely valid to play D&D as a grim survival game where your characters are poor, starving retches with only the barest hope of survival, but if you want that then 13th Age is clearly not what you are looking for. And I suppose you might find the sidebars and the jokes off-putting or or lacking gravitas, and if so then 13th Age would have little value for you. I recognize that the biggest part of the reason I like 13th Age is that my sensibilities happen to line up pretty closely with Tweet and Heinsoo. As I said regarding Midgard, 13th Age is not what everyone is looking for, but it is definitely what some people are looking for, and in this case "some people" includes "me."
But it is more than tone that 13th Age brings to the table. Mechanically, 13th Age was being designed around the same time that 5e was being worked on, and I think you can detect a very similar set of design goals. First, both 13th Age and 5e start from the proposition that the two (at the time) primarily competitors in the d20 space--Pathfinder and 4e--are too complicated and play too slowly, especially in combat. As a result, both games significantly pare down the volume of rules and strip away much of the tactical dimension that was front and center in Pathfinder and 4e. The overall complexity level of the two games is more or less the same (with the possible exception of the magic and spellcasting rules, which are simpler in 13th Age than in 5e). In many ways, 13th Age is a kind of "mirror universe" version of 5e, in the sense that it shows off a different set of solutions to some of the same fundamental problems that 5e was grappling with.
Having said that, I think 5e is in many ways a more conservative design than 13th Age. By virtue of not having to be all things to all sorts of D&D players (which is the defining feature of 5e, and which I will get to in the next post), 13th Age is more free to innovate and experiment. Notably, in part because 13th Age doesn't have to directly react to the backlash to 4e, it keeps more of the good ideas that came out of 4e and improves on them. Monster design in 13th Age is even more 4e-like than 4e, in the sense that they have few stats to keep track of and are very easy for the GM to run on the fly, making it much simpler for the GM than 5e. 13th Age also innovates in terms of class design, in that there are a host of classes that have unique and interesting mechanics that are fun to play (the Fighter and the Rogue are particularly good from the core book, as are most of the classes in the expansion 13 True Ways). 13th Age also basically removes the non-magical shopping mini-game of previous editions, where you figure out how many coils of rope you can buy or whether it is advantageous to use a falchion or a glaive-guisarme. Instead, armor and weapon damage are basically derived from your class and can be flavored however you want, while mundane equipment is hand-waived away.
But the biggest point of departure is the way that 13th Age merges concepts from narrative and story-oriented games into the d20 framework. Before working on 3rd Edition D&D, Jonathan Tweet was a designer for Ars Magica and Over the Edge, two games that in many ways are ancestors of the "indie" narratively-focused games that began to come out in the early 00s. This background positions 13th Age to take parts of that strand of game design and splice it back into the basic D&D framework. This splicing happens in a couple of ways. First, 13th Age works on the notion that players will have significant agency in story creation, rather than putting all of that on the GM. At character creation, players select backgrounds (descriptions of pre-adventuring backstory that take the place of d20-style skill systems), Icon relationships (positive, negative, or conflicted connections to the major NPC forces of the world), and a "One Unique Thing" (some detail that is true only for that character). These elements work to build out the world--if a player decides that their One Unique Thing is that they are the only survivor of the Knights of Nee, then the Knights of Nee are an thing that existed in the campaign world.
Facilitating this sort of world building is the default campaign setting of the Dragon Empire, which is very deliberately only half built-out. You have a map with some city names and some geographic locations, but there is no more than a paragraph of description in the Core book for each place. I was initially somewhat perplexed by this (expecting a hyper-detailed campaign setting of the type you see in something like Midgard Campaign Setting), but it dawned on me that the ambiguity was designed so that the players and GM would fill in the details as they went along. It's similar to the way games like Dungeon World tell you to "draw maps, leave blanks"--while 13th Age draws the map for you, there are plenty of blanks to fill in. Even better, later supplements routinely present a series of sometimes mutually exclusive options for the GM to pick from--the Bestiary gives five different backstories for dark elves, 13 True Ways has city descriptions with multiple options, etc. By approaching world building in this way, you never have to worry about awkwardly shoe-horning your campaign into a pre-designed world, but you also have a platform to build the campaign around, as opposed to building out of whole cloth.
What this sets up is a two-phase play experience. When the game is out of combat and the players are exploring or talking to NPCs, 13th Age plays like a narrative game--something not all that different from Dungeon World. But, when combat comes, 13th Age snaps back to being a D&D style game, albeit one that is simpler and more abstract than Pathfinder or 3e/4e. For me, this is the sweet spot between the two styles of game. One of the things I have found for more rules-light or narratively focused games is that combat is often the least interesting thing you can do in those systems (that's definitely my experience with Dungeon World, and it is also my experience with Cypher System games like Numenera). That's fine for certain sorts of games where combat is not a big part of the story, but if you are going to do a D&D adjacent thing, you need combat to be engaging in a way that I don't personally find true in most narrative systems. On the flip side, if you watch carefully streams of D&D like Critical Role, there are long stretches of play where there is very little rules engagement, because there is little mechanical support in traditional D&D for interacting with NPCs or exploring backstory. 13th Age doesn't go as far as some narrative games, but it provides a number of mechanical hooks and pieces to interface with those portions of the play experience.
So, I love 13th Age because I think it sits perfectly in the middle of D&D style games and more story-oriented games. But there are a couple of things to point out that might make it a poor fit for groups and GMs. First, it basically requires an experienced GM, and more specifically an experienced GM that is good at improvising. Again, to be fair the text says that right up front, but juggling the different Icon relationship die on the fly would be challenging if you are a prep-oriented GM. That fits well into my style, but it's not everyone's style. Also, having run a number of one shots for 13th Age at conventions and other situations, I think that those settings don't do a good job of showing off what makes 13th Age special. It is really hard to work the Icon relationships and One Unique Things into a one-off session, and without those things 13th Age is basically an alternative version of 5e without much that sets it apart. Longer campaigns, or at least multi-session stories, are really the preferred home for 13th Age.
But I think the biggest thing about 13th Age, to return to what I started with, is that it is an opinionated game. Among the various ways that D&D can be expressed and played, 13th Age picks one and runs with it. If you are on the same wavelength as the designers and play a game with that sort of tone and sensibilities, then you are good to go. But 13th Age doesn't work as well if you want to do something radically different, like gritty dungeon crawling. 13th Age gives you enormous flexibility within the context of big epic fantasy, but it really only does big epic fantasy. Don't fight the game's design goals and perspectives--if it's not what you want, go find a different game, of which there are many options.