What Magic Would Be Most Realistically Most Impactful?

[Celebrim]

1) I don’t have my DMG handy, but I’ll note none of the rules you just cited bar class entry, just advancement beyond a certain point. It would be interesting to see if a reason or broader context is offered for why their progression is capped. I doubt one is, but if one exists, I wouldn’t be surprised to see it based in mechanics or just by proclamation...probably for game balance reasons. I would also note that a good bit of the material published in campaign settings and modules didn’t follow demographics & limitations put forth in the DMG. Even the line about the majority of people being zero level types is subverted by products like T1 Hommlet.

I suspect hardly anyone actually followed the DMG demographics, including Gygax, which seem to imply things like only 2% of humans are 0 level man-at-arms - much less classed. The hard line regarding demographics taken by the DMG was I suspect entirely done to give DMs teeth with respect to resisting plans by PC's to do the sort of things you are suggesting or otherwise raising armies that took the focus off the heroics of the PCs. As with most of the seemingly adversarial advice in the DMG, I don't think Gygax means the DM to follow it strictly, but simply means to set a standard so harsh that any rules lawyerish PC familiar with the rules will have no grounds to complain that their DM is being overly strict. The real underlying meaning of it all is "Don't be a pushover."

(FWIW, I seem to remember a bit of demographic breakdown in the DMG that covered mundane professions and classed individuals, but I could be wrong.)

There is to my knowledge no direct guidelines for figuring out how many classed individuals there are in a setting. The write up of henchmen certainly implies that NPCs capable of advancing in a class are rare, and it gives a rough percentage of the population that are potential henchmen and you might could perhaps infer something about the total percentage of classed NPCs from that but it would be all guesswork. There is a table of attributes for normal humans that suggests that a fairly high percentage of the population is less than 0th level, but it gives no percentages that fall into the various classes. The rules on rebellions imply about 1/5th of a population at most is capable of taking up arms, and that of that if trained about 10% (2% of the total) will become full 0th level men-at-arms (4-7 h.p., 0th level fighter THAC0).

But all of that is just guesswork and will involve various assumptions. Again, I think that's probably deliberate if the real purpose of all this is to prevent players of PC lords from dictating to the DM what sort of characters exist in their dominions, or from being able to pursue a program where they tell the DM that they set up classes to teach spellcraft to the population and turn every NPC with 9+ INT into a M-U. If you look at demographics as they actually existed, as you point out, now town, village, or hamlet is so restrictive.

Some idea of what the normal percentage of wizard combatants was expected to be can be inferred by the description of armies of 'Men' - whether bandits, pilgrims, pirates, or what not - in the Monster Manual. I don't think you can find support for the concept of armies of low level M-U's in AD&D. But you are free to develop whatever demographics and evolve the nature of warfare however you like.

2) AD&D also has NPC classes, including casters like the Witch, Shaman, Incantrix and probably others, detailed in TSR’s official periodical, Dragon. IOW, there’s probably more casters out there than are usually accounted for.

Perhaps. The Sage in the DMG although not explicitly a class nonetheless has pretty decent hit points and some ability as a caster (up to 6th level spells if I remember correctly). But Sages break the PC rules for spell progression and caster level and all the rest really are probably so rare as to hardly be worth mentioning, and in any event given the prima donna nature of the Sage as described in the entry they certainly aren't going to be roped into joining the army.

I have very extensive experience with mass combat in 1e AD&D. Any high level character is capable of matching up with veritable armies in AD&D. A 16HD elemental on the battlefield is something that I've definitely seen in a non-theoretical manner. What tends to happen is that essentially the high level characters are busy countering each others moves. If you have some M-U sitting back behind the battlefield and their main move is going to be conjuring an elemental, then either you have M-U's on the other side that focus on that elemental in some fashion, or the high level fighters focus on the elemental. That leaves the rest of the army maneuvering against their army in some fashion. The question of what warfare evolves to in a world where high level characters are worth scores or hundreds of low level characters is not an easy one to answer, and mostly comes down to, "What do you want this to look like?"

 

[Celebrim]

I can't count the number of player characters I've seen use Charm Person flippantly over the years.

It's a bad enough problem I have to explicitly warn any PC spell-caster before they start playing. I have no idea what it is about the spell that causes players to act in that manner, but the very fact that players do act in that manner strongly suggests how terrible the temptation of the power would be if it were real and conversely how profoundly hated the power would be to those that felt exposed to it. Players invariably seem to look at it as some sort of harmless prank.

Magic that controls the mind is considered one leg of the tripartite 'black magic' or 'witchcraft' in my game along with necromancy and "diablerie" (summoning or communing with evil spirits). The penalty for charming or dominating someone is the same as the penalty for rape, which is generally death by being drawn and quartered. "He tried to mind rape me." will definitely get attention from a community, and if the PC isn't held in some esteem may lead to a lynching.

 

[doctorbadwolf]

There is an element that we'd need to address in discussing this, that most RPGs do very poorly: demographics.

How many wizards/druids/clerics/users of any particular power are there in the world? That puts a bound on how impactful a power is on a broad scale of a society.

There are actually two questions there.

1) How rare does the setting "want" those things to be?

2) Can any of the types of magic be taught to anyone? Sorcery is inherent, but can wizardry simply be trained? What about pact magic? Could a warlock who gains political or mercantile power train others to make the same pact and create a nation with FeyLock knights patrolling the land to protect the populace? Could a wizard create a public school of wizardry? Can a Bard college teach you how to channel the power of language and song to create wonders even if you have no inherent knack for it?

Some DMs and writers will begin from question 1 and use it to answer question 2, while some will go the other way around.

I prefer to determine the magic of a setting by answering question 2, and then make a world that makes sense to me from that premise.

If wizardry, bardic magic, pact magic, and even divine/druidic magic can be taught to literally anyone with a will to learn it, then we can reasonably assume much greater proliferation of magic, magic items, enchanted infrastructure, etc.

But what that looks like changes dramatically if only wizardry can, or if pacts are easily made and there are entities that are friendly seelie fey or hexblades can be forged by mortals, or if druidry or divine magic can be learnt by anyone of faith with fellows to invest them, or if learning Bardic magic is as easy as learning to be a bard in real world history (so, years of study, but no magical spark required).

If wizardly ritual casting is something that simply takes a decade of learning, that's gonna be as common as blacksmiths. Just those level 1 rituals are very powerful.

 

[doctorbadwolf]

I'd go for something like Zone of Truth or other magics of that ilk. Again, with the acceptance and potentially commonality, I think it would revolutionize human's justice systems and politics.

Command is just as powerful, in the same way. "Confess."

Plant Growth. A single 8-hour casting enriches all plants in a 1 mile diameter for a year, yielding twice the food. Think about that in a lot of countries, especially ones where distribution is hard. But really, you mentioned healing - with all of the healing, cure disease, regeneration, and raise dead we might reduce early death that having more food in the same area is needed.

Mold Earth is an even bigger deal, since it's a cantrip, and can create irrigation with vastly less workforce than has been required IRL. Also, just Detect Poison and Disease would revolutionize medicine in ways that would reach far beyond those people who can use magic.

Plant Growth is the most impactful spell in 5e D&D.

EDIT: At least in the core rules. The cantrip Mold Earth from Xanathar's Guide to Everything beats it.

Creating water from nothing, in the hands of someone who is being paid to just do that as many times a day as they can, every day, for a year, is world changing. 1st level spell, 10 gallons of water per casting is thousands of gallons a year from one person. A druid who can cast Plant Growth can also use Create or Destroy Water and Mold Earth to impact the irrigation beyond that 1 year of doubled crop yield.

Remember that while a modern American can use dozens of gallons of water a day even while trying to conserve, a pre-modern family will probably use less than that 10 gallons per day. If you're looking at a dry year, rather than a nearly waterless place, creating better irrigation and water storage, and supplementing the water supply, can save a community just as much as increased crop yield.


But let's get weird! Purify Food And Drink.

What is "drink"? Is a well full of soiled water "drink"? Can you filter out the physical matter from soiled water, and then Purify that water in 10 ft diameter spheres? Can you use Mold Earth to make an estuary river from the coastline to fill small pools that can then be purified into potable water?

Does the spell make food waste no longer a concern? Does such a world even still have the phrase "A few bad apples spoil the bunch"? Apples can be stored for over a year in the real world, using no modern technology. What if 1 spell casting per year makes them last forever? What about meat?

What impact does "Detect Poison and Disease" have on managing livestock? What sorts of injuries are simply cured by cure wounds? How many livestock animals have to be put down because of relatively simple injuries and diseases, not to mention mysteriously dying because they ate the wrong thing, which the spell would identify. Protection From Poison is 2nd level, but Bless gives a bonus on saving throws, which combined with an informed use of veterinarian medicine will dramatically increase the survival rate of humans and animals.

 

[doctorbadwolf]

That's the cost of a car. And then, it's not "never be made uncomfortable by the weather again", it's "one specific set of clothing you won't be uncomfortable". So it needs to be washed. Is it work clothes, party clothes, outdoor clothes - it's not all fashions (that's a different enchantment). Most people also won't wash and wear the same clothing every day to every event.

In 5e, the "temperate" enchantment is a minor effect that can be added to any magic item without impacting it's rarity, according to the DMG. XGTE, IIRC, states that these DMG minor effects make good common magic items.

So, one could simply have a ring that makes you comfortable in any weather but the absolute most extreme.

About Charm Person, since someone brought that up. Mind control magic would definately be a big deal, though I go by the 5e version which is absolutely not mind control. You can't make anyone do anything with Charm Person in 5e, you just make them treat you as a friendly acquaintance. Which would probably still be illegal, but hardly gonna get people murdered in their sleep for being capable of it.

Dominate Person would, in a just world, lead to the harshest punishment allowed by the laws of the land.

But still, most places wouldn't go around murdering people in their sleep because it's known that they can do a bit of enchantment.

 

[Umbran]

Command is just as powerful, in the same way. "Confess."

Thankfully, that won't work in a meaningful way - it doesn't stipulate what they have to confess to. "I confess that I think puppies are adorable!" fulfills the Command.

 

[Blue]

Command is just as powerful, in the same way. "Confess."

Legal
Prosecutor: Confess!
Innocent person: Confesses (as forced to do by the spell).

Political
There's no way this can let me hear debates, state of the union, and everything else and know where they are intentionally lying. Including the "do you have any reason to think people are feeding you false information so you think it's true". Discussion at federal and state levels in the US.

Nope, still up for Zone of Truth. Note that I didn't say telepathy for these as well, because you need to trust the telepath.

 

[Aldarc]

The very first aircraft carrier changed warfare. Once one was fielded, everyone realized their potential and either started research (if they hadn’t already) or working out how to neutralize them.

Jein. Some people realized it way before others. Others still felt that the battleship was still the ship supreme for naval warfare. It took awhile, even in WW2, for the true impact of the carrier to be recognized by the various nations. I don't think that it was as obvious then as it is now in retrospect.

See also Harry Turtledove’s Darkness novel of a (non-D&D version) WW2 style conflict in a fantasy world.

That author has way too much idle time, both real and alternate, on their hands.

 

[Dannyalcatraz]

The first carriers hit the waves in the 1920s- first the British, then the Japanese. By the mid-1930s, all of the major sea powers operated several, mostly with airplanes specifically designed to be flown from carrier decks. Usually with folding wings as well.

The powers that had them in numbers understood their power quite well. It’s not an accident the way the strike forces that the Japanese and American military deployed in the Pacific were so carrier-centric, or that the Germans tried to trump them with U-boats in the Atlantic.

 

[MGibster]

Shadowrun does a decent job showing how magic affects the economy. The game is hyper focused on runners, so you typically see mages and shamans use their skills in the pursuit of skullduggery and violence. But in the wider world they're also involved in entertainment and major corporations operate magic shops like it's just another consumer product.