Pathfinder 2E for grimdark

[Supes]

Obviously, PF is high fantasy and the bonuses that you gain for magic items and spells and such seem to be baked into the balance formula, but what things would you change, what rules would you modify, what spells would you eliminate or restrict, and what items would you allow (maybe an easier way of looking at it)?

A couple of changes that seem to make sense would be that your dying condition value would become your wounded condition value when brought back, and wounded 3 (or more) would result in a permanent deformity. Additionally, I would make +1 potency and striking runes (or +1 potency and resilient runes) more like the old exceptional quality or masterwork items and the +2 and greater true rare magic items. Limit magic shops and formulas for higher level magic items.

What other things?

[Porridge]

Using the Automatic Bonus Progression rules would allow you to eliminate a lot of the high powered magic items without throwing the math out of whack.

If you also wanted to power down the PF2 high fantasy feel, you might forbid PCs from taking spellcasting classes as their main class, but still allow people to take spellcaster class archetypes. That would limit the level and number is spells available, and tamp down on the high fantasy feel of the game.

[Castilliano]

Some of the older versions (like 1st ed. D&D old) laid out a similar pattern for magic weapons (despite them all being magic and requiring risky endeavors to create).

But back to grimdark, I think it all depends on what one means by grimdark. Ravenloft was certainly grimdark, yet it had high fantasy levels of magic (albeit with spotty access) while Conan & Cthulhu didn't. Or there's the Midnight 3.X RPG setting that made ANY magic other than a really weak consumable rare. It was ultimate grimdark in other ways too if you want an extreme example to work off of.
Then there are the science fiction (or Shadowrun) games where the equipment & market exist, it's just the PCs will never afford it while their worst enemies can. (That just sparked an idea that maybe magic gets embedded like cyberware so that it's often irretrievable or retrieved through gross extraction methods. "Hey, if I have to eat his heart, I'll eat his heart.")

So part of this amounts to how gritty you want your grimdark, how self-sufficient (or insufficient) you want the heroes to be, and of course the style of story you tell (regardless of mechanics). What will the role of NPCs be, and how (in)sufficient will NPC resources be?

Personally, I think attrition, loss of agency, and unreliability make for solid principles re: grimdark. This extends to the non-enemy NPCs too. Of course a burst of light every so often would contrast how very dark situations are.

So yeah, permanent deformities work well (or any permanent or long-term effect for that matter). I'd also make it harder to get rid of the Wounded condition and/or make the Doomed condition more frequent at lower levels.

-Attrition: Medicine would have to outright be nerfed (but see below) and Conditions a bit stickier, i.e. extend the effects of poisons. Limited avenues of recovery could help with this.

-Loss of Agency 1: A hard one to finesse since people play RPGs to have PCs that can and get to lots of cool things, but...
Insanity/horror rules might apply depending on setting, with such mechanics used in Ravenloft, Deadlands, and Cthulhu so they're pretty popular (or notorious?). It's like "damage to personal agency".
Losing one's humanity could be another factor (perhaps with fringe benefits, but ultimately the loss of the PC.)
So maybe look into how those systems did it if interested.
Heck, when you think about it, there's a lot of horror in high fantasy, it's just approached as if the hero will handle it and the kingdom will bask in sunshine.

-Loss of agency 2: High fantasy heroes flourish, often with much gained along the way in terms of friends, honors, and wealth; grimdark heroes survive, hoping they've kept hold of what little they've got. That's a storytelling point more than anything, but I was reminded writing this of Ravenloft advice saying in one module that survival should be the PCs main reward!
In that vein, and that of the RPGs & settings previously mentioned, there should ALWAYS be villains around that the heroes simply cannot and maybe never will be able to eliminate. Thwart the plans of, sure, but not want to confront even though their always present in some form. :O

-Unreliability (& less agency): Part of the hero/superhero motif is controlling their environment, with the strongest dominating it (think non-street-level DC heroes). Their struggles vs. their enemies isn't significantly different, but as well as stakes, one's power level changes how one can tackle the environmental obstacles.
In grimdark, I'd crank that down and here's how:
Take level out of the equation for proficiency. For combat balance this also means subtracting the monster's level from their stats too, but this change makes the static dangers (i.e. climbing or balancing) remain relevant for longer AND makes hordes that much more dangerous. It flattens the playing field a lot, which unfortunately makes it easier for PCs to punch upward at your archvillains so account for that. But the PCs will seldom be able to say "Remember when we used to be afraid of Ogres? Ha, ha ha!" because those minor monsters will continue to hit and do okay against AoEs! (though they'll need to come in greater numbers)
The side benefit of this is Medicine (if you keep the original DCs) will become less and less useful as a percentage of PC hit points so they'll have to spend more and more resources healing (assuming you're putting pressure on them in terms of time or enemy presence).
Just beware that attrition will make final battles more skewed!

Oh, and the feeling of being trapped matters. Whether place, personal situation, etc. As long as the PCs have enough agency to bounce off the walls (and occasionally with an unexpected breakthrough so they keep pecking for the pellet if you know that psychological experiment).

[Ascalaphus]

I don't really associate grimdark with equipment so much. Rather, it's about whether stories have happy endings.

Typical fantasy: you slay the dragon and the world is better for it. Maybe marry a prince(ss) and inherit the kingdom.

Grimdark: you put a stop to the serial killer but not before many people were killed and others scarred. Instead of thanking you, people are angry at you for all the people you were too late to save.

[Castilliano]

Ascalaphus wrote:

I don't really associate grimdark with equipment so much. Rather, it's about whether stories have happy endings.

Typical fantasy: you slay the dragon and the world is better for it. Maybe marry a prince(ss) and inherit the kingdom.

Grimdark: you put a stop to the serial killer but not before many people were killed and others scarred. Instead of thanking you, people are angry at you for all the people you were too late to save.

To keep with the dragon theme + grimdark:

You slay the dragon, but it's bound its soul to the maiden you've rescued and you have to kill her too or the dragon will respawn.

You couldn't slay the dragon because DRAGON, yet you managed to rescue the maiden. Who has Stockholm Syndrome and returns (or betrays you).

You slay the dragon, unleashing a wrath against humanity by other dragons or maybe only its more powerful parents(s). Maybe even loses you a good dragon ally.
Or turning it into an undead dragon, a font of elemental creatures, or some other "worse thing than before".
Or its essence is what warmed the region so an ice age returns.

It's treasure is cursed, yet you don't realize until after you've spent a huge amount that's now spread into the world's markets...and you'd better track it all down or (insert bad things).

---
I'm reminded of a player who asked me for advice roleplaying his 3 Charisma* Dwarf who had good intentions. Chaotic or evil & uncharismatic is easy, but good (maybe even lawful)?
My advice was that no matter how hard the Dwarf tried to be good, civil, nice, or whatnot, he simply had to end each conversation or social encounter with the other person thinking less of him. In that light, RPing became easy-peasy for the player.

Applying that thinking to grimdark, no matter the PCs' heroic efforts, the world becomes lesser/grimmer/darker despite their successes. That's not necessarily by nixing their success itself, perhaps only by contrast, revelation, or a separate situation arising. Of course, this negativity is why grimdark doesn't suit all or even many tables! Set expectations early!

*We'd rolled and his other stats were phenomenal, best in party! That 3 eventually soured him to the character though so it did its job. :)

[Rushniyamat]

I recommamd exploring the OSR for inspiration for rules and changes. One good idea you can burrow from them is making monsters dangerous and deadly, but adding a lot of freedom to the players' approach to battles: ambushes, allowance of clever use of the surroundings, and more. Also, running away should always be a valid option.

If you want to keep magic, make healing spells dangerous in some way. Maybe they attract demonic influence, require dark methods or are dangerous to the user. Forbid teleportation spells (but teleportation rituals should still exist. And add consequences for performing rituals). Furthermore, you should find an in-world explantion for why magic is rare.