But what role are good-aligned monsters meant to have when it comes to game design? One could argue that they serve as allies to the party, particularly when viewed in the case of summoned monsters, but that set of spells hasn’t been universal throughout Editions. And in regards to the more explicitly powerful entities like ancient metallic dragons, storm giants, and solar angels, they can become a minor version of the Elminster Problem where one asks why they haven’t gotten around to dealing with some problem or other that the PCs must handle. Another argument I’ve heard is that they can be an alternative set of antagonists in campaigns where the PCs are evil-aligned or otherwise morally compromised, but evil campaigns tend to be rather rare. And the amount of good-aligned monsters that exist typically aren’t large enough to match the versatility and scale of the rest of the monster manuals.
So, what purpose do you think good-aligned monsters serve from a mechanical standpoint? I’m eager to hear people’s thoughts.
I think I need a bit of clarification.
If the question is
why are there stat blocks for Good creatures, the answer is, primarily, so that they can be fought. Not all PC's are Good, not all "Good" creatures are benevolent or helpful for the PC's goals. Good creatures can make for some interesting and nuanced antagonists; even a straightforward "fallen angel" storyline can be a lot of fun, and a more nuanced "Good, but Antagonistic" enemy for the party is just meaty as heck with role playing and interesting combat opportunities. Sometimes the PC's are just treasure hunters, and a metallic dragon's hoard is worth as much as a chromatic's, after all. Solar angels may serve as guardians of ancient ruins.
Secondarily, Good creatures have stat blocks to be allies in combat. This is probably one of the bigger design goal misses in D&D's history, but ideally, you have stat blocks that are good from a Player perspective to call on, with abilities like healing and buffs that make it a good ally to use in a fight, with Player-facing mechanics that call upon the creature. Being an angel-summoning agent of divine will is a fun fantasy archetype that relies on these stat blocks.
In both cases, you use stat blocks to fight. Stat blocks are primarily a combat element, so using them to fight PC's, or for the PC's to use in a fight, is why you would have stat blocks for Good creatures.
This is enough reason to have most of the Good creatures in the MM, for me.
But there's a more broad question, which is
why do Good monsters even exist in the game? As in, even if the Solar had no functional statblock and was never anything you fought or called upon to fight for you, what does it do?
Well, without a Combat dimension, we can look at the other pillars - exploration and social interaction.
The latter pillar is pretty straightforward, because it's the same reasons you'd give a Good character a stat block - because they make for very interesting antagonists and fun allies. Not something you want to use all the time, of course, but when you want your D&D game to get a little philosophical and pit versions of Good against each other and show how easy it is to stray from the path and fall (even for these exemplars), that's a juicy critter to use as an antagonist. And when you're recruiting powerful allies against the armies of evil or something, they can be fun allies. Even if you don't directly use them in combat, they can be a fun way to counter an overwhelming enemy force with an overwhelming ally force (every red dragon is opposed by a gold, every cloud giant is opposed by a storm giant). So they're extremely useful on that pillar.
The other pillar - exploration - is a rarer use case, but not absent. Powerful Good monsters are often at the center of lore-heavy locations in the world, such as ruins of kingdoms past watched over by angelic protectors or the resting places of dark artifacts guarded by metallic dragons. These creatures give those locations a voice, a defender, and a thing to find that adds to the location's characterization.
So, broadly, in terms of game design:
- Good Monsters make interesting antagonists, to fight or to deal with in ways other than fighting (such as convincing a metallic dragon to give up an artifact of great power in its hoard to a party of grubby adventurers)
- Good Monsters are great PC allies, either in a fight, or in ways other than fighting (such as calling upon the wisdom of an angel to resolve a dispute between powerful kings)
- Good Monsters add depth and characterization to locations in the world such as ruins and abandoned temples and other kinds of dungeons (such as the lair of a reclusive storm giant). They can make it easy and fun to interact with the lore and history of the setting.
They add things that are a little less direct, too, such as worldbuilding and exploring the dimensions of the game's conflicts, but those aren't quite as critical.
FWIW, I think it's wrong to imagine the MM as a collection of statblocks, and even more, it's wrong to imagine the MM as a collection of statblocks that you are intended to fight. 4e's design was colossally disappointing in that regard, even as it was very focused for its intent.
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