Ahem. I was there.
In CHAINMAIL there were wizards that functioned as artillery.
Then there was Dave Arneson’s first miniatures/roleplaying campaign. Some players were ‘good guys’ and some players were ‘bad guys’ and Dave was the referee.
One of the ‘bad guys’ wanted to play a Vampire. He was extremely smart and capable, and as he got more and more experience he got tougher and tougher.
This was the early 70s, so the model for ‘vampire’ was Christopher Lee in Hammer films. No deep folklore shit.
Well, after a time, nobody could touch Sir Fang. Yes, that was his name.
To fix the threatened end of the game they came up with a character that was, at first, a ‘vampire hunter’. Peter Cushing in the same films.
As the rough specs were drawn up, comments about the need for healing and for curing disease came up.
Ta da, the “priest” was born. Changed later to ‘cleric’.
The bit about edged weapons was from Gary’s reading the old stories about Archbishop Turpin, who wielded a mace because he didn’t want to shed blood (“who lives by the sword dies by the sword”).
In other words, it came about the same way that 90% of the D&D rules came about :
WE MADE UP SOME SHIT THAT WE THOUGHT WOULD BE FUN.
Here’s my take on this for a campaign setting.
You’ve got Fighting-Men, Magic-Users, and Clerics. If Clerics came about to fight Undead, maybe the other classes came about to fight specific anti-human (Chaotic) forces. Magic-Users seem to have a lot of planar spells and demon-related stuff, so let’s say humanity created Magic-Users to fight demons. Let’s take a step further and say that Magic-Users actually use demon-magic against them, which is why we never had M-Us before we came across demons.
So we have a campaign history where humanity, whose great strength is in adaptability, brings forth specialized heroes to combat each new menace as it appears. Let’s say Fighting-Men existed to fight the evils of this world or maybe specifically dragons (good save vs. breath weapon!). Then demons came along and we bootstrapped ourselves some M-U demon-slayers. Undead make an appearance and we get Clerics.
Demihumans like Dwarves and Elves and Hobbits can’t be all these classes. Dwarves and Hobbits can be only Fighting-Men, while Elves can be F-Ms and M-Us. Let’s say Elves can be M-U because of some connection with the demons: maybe they fought the demons long ago and actually helped the Humans develop their skills when demons resurfaced. This is also why Elves are limited in how far they can advance as M-Us, and why other races can’t be very high level F-Ms. Classes are for humanity, to represent our adaptability, and demihumans can do it too (which is what defines them as demihuman) but to a limit. Elves don’t get bored with wizardry; it’s just not in their nature to be able to advance further.
Now a new threat has emerged, parting the veil between the Planes and insinuating itself into the world of humanity. How will we fight them? Our old tactics don’t work, our old magics aren’t effective. Hints and rumors of the existence of these creatures lies in the oldest ruins and catacombs, and artifacts once ignored as useless may have some impact. This is where we begin the campaign.
Here’s how I would approach this. Either threat first, or class first. If you create the threat, you need to create a class that will have skills to combat it. Use the existing classes as a guide. A Cleric for example can Turn Undead. He has other spells but the turning is why he exists. M-Us can Teleport, Banish, Dispel Magic, Lower Magic Resistance, etc.
This means there isn’t much reason for a Thief, Ranger, Barbarian, etc. to exist. We’re not talking about a class filling some task-related role (else we would have a Chamberpot Emptier and a Mink Coat Maker class). Nor a specialist M-U, since M-Us are already here. An Alchemist should already exist and there’s hardly any reason for some external threat to cause them to emerge. A Paladin is basically a Cleric/Fighter. Almost all Fighter variants are excessively situational, dumb, and unnecessary because Fighter still exists OR they render Fighter useless because they’re better.
This requires actual creativity. It isn’t the opportunity you’ve always waited for to trot out your Jedi clone.
I could see an argument for a Psionicist class since that’s always been pretty alien and if it’s based on Psi Strength Points instead of Vancian memorization the class operates differently from the existing ones. It’s also a nice choice because there is a lot of existing Psionic material out there and hardly anybody uses it because almost without exception you need to rework it for game balance. The new threat can include psionic monsters in D&D that nobody uses because Psionics is a pain in the butt to manage and it’s too powerful to hit non-Psionic PCs with unless they’re well over its HD.
There’s another caveat. Let’s say you start with a class like Bard because you like Bards and you want them to finally be useful and not completely stupid. The threat you create is going to be one that you can harm by singing at it, which is just as ridiculous as the Bard is anyway and it’s a thinly-veiled attempt to validate the Bard’s existence.
The new threat has to be level-appropriate too. Don’t just make the new threat +10HD and 100% MR. I’ve seen that happen and it’s loathsome. I loathed that campaign. Note that there are 1 HD demons, 1 HD undead, so your new threat should have degrees of opposition from 1 to 13 or so. Another problem is that this process can easily make the other classes feel useless. It’s important that the threat be hittable by other classes but it’s just too dangerous and/or ineffective to do it regularly. Maybe a F-M hitting the new threat feels like he’s as effective as he would be at half his level.
I think a more fertile way to develop this is to start with the threat. Really go nuts, look for inspiration anywhere, and then start beefing it up to get the full scale of 1 HD – 10 HD. Give them special abilities and defenses, ecology and maybe society. Then think about what parts of this threat are really irritating for a PC group and the counters to those. These are the abilities a PC will want when fighting them.
And don’t be afraid of being too tough! Remember, in 0D&D Undead who drain levels drain them permanently. There is no Restoration. By the next edition we had developed our Clerics’ abilities to the point where the most fearsome Undead ability can be mitigated. That might not happen until well into the Fourth Class campaign, or even after it!
Don’t stop using other monsters. Remember these other classes are useful still! There will still be demons and undead all over the place and the new threat might employ them or just happen to coincide with them.
Star Trek did this already. First you have Klingons which only a couple Starfleet officers can take on in hand-to-hand. They’re tough badasses. By Second Generation we have a Klingon officer on board and basically anybody can fight one. Then we encounter the Borg, which are a huge threat. By Voyager we have a Borg officer and kinda anybody can take one on. They turn into sad little zombies. Plus we have all the shift-phaser stuff to get past their special shield tech. Villain Decay is what the cool kids are calling it.