Common enough to be a playable race. Also, really? Orcs, the most iconic of villains? And you want to play the "they might not be common" card? They are stock humanoids.
100%. I've been in homebrew settings where they don't even exist. They also didn't exist on Athas. They might not in fact be common in any particular game.
Are you serious right now? Elf is in the PHB. Human is in the PHB. Knowing that they don't have any R's, I's or V's is not metagaming. Heck, you can't even use the "how common are they" because the answer to how common humans is is "Yes". They are the single most common creature in the entire game.
Way to ignore the other three races you lumped in with humans and elves. Very telling that.
As for common. They too might not even be in any given game. Or might be exceptionally rare and not a PC race.
I don't know Max, why might I assume a creauture made of MAGMA (a fluid) might be difficult to smash or cut. I'll also go out further on a limb and guess a creature made of MAGMA might not burn. Wild guess, but it seems kind of logical.
Magma is a very thick fluid. It would seem to me like a mace might just scoop a large swath of the body out and flick it away doing extra damage.
And they eat. They are like Jellyfish. They do have a functioning digestive system. Skeletons don't. Zombies don't. Animated suits of armor don't.
So now you're limiting it to undead?
Also, let's say that my character assumes oozes ARE immune to poison. Is that meta-gaming? Am I going to get called out for not using poison on a creature I incorrectly assume is immune to it?
No. Nor is every case of assuming something that is true a case of metagaming. As I said, if it makes sense for the PC to make that assumption, that's fine. If you have to look for a weak justification, you've walked into metagaming territory.
Right, so when the PCs learn it once, they learn it forever. How useful is an ability that is only needed once? We generally call that "niche" right?
Once per type of monster isn't niche. Sure, if you could only use it once in your PCs life on one monster, that would be niche. DMs use many, many different kinds of monsters, though, so even if you don't personally like it, it will be very useful.
Why would someone assume that a Giant living in the Tundra and covered in ice is going to be the same as a giant living underwater and throwing lightning bolts? Hey, I bet the guy who can summon and throw lighting as just a natural part of his existence doesn't take full lightning damage.
You mean that giant wizard?
Also, they may not be demons, but they are extraplanar beings of pure evil, formed in the lower planes. That seems like a really similiar thing. Kind of like how orcs and goblins and humans and elves are all humanoids from the material plane, and all lack natural resistance to fire, cold, and lightning.
So creatures that are from three wildly different planes are like creatures who are all from the same plane?
Also, what do you mean that assuming can get me in trouble? Is attacking a Demon with a silver weapon going to cause me to explode? No, it just... won't be as effective as I thought. Oh no! That would mean... well, since I had to go and silver my weapons I probably didn't have access to magic weapons. So at worst that just means I spent money on an upgrade that doesn't help. How is this bad? I guess I could have spent the money on something else, but it wouldn't have been anything that helped in the fight.
Assuming incorrectly can bite you in the rear. I wasn't limiting that statement to demons and silver weapons.
What counts as a good in-character reason then?
It varies. It could be as simple as something in your background about growing up near the troll moors or an uncle who was a troll hunter. It's very probable that he would know about troll regeneration and what to do about it.
Ah, so you want to cheat. If an ability says that it tells you Vulnerabilities, then it doesn't mean you get to learn their special traits. It means you get to learn their Vulnerabilities. Of which trolls have none.
I'm the DM. I can't cheat. If I want to allow the troll's vulnerability to fire and acid to be revealed by a power that informs a PC what a monster's vulnerabilities are, I can.
If you want to have an ability that does tell you their special abilities then we are talking about something completely different. I've actually advocated for the Ranger's new Hunter's Lore ability to basically give the player the monster's statblock. Learning AC, HP, special abilities, average damage, Vulnerabilities, resistances and immunities IS actually useful, because that information changes and is useful every fight.
AC, HP and average damage are not in-fiction things for a PC to learn. Those are OOC things. Special abilities, vulnerabilities, resistances and immunities are.
