Swarmkeeper
Hero
In 5e, if you deem something is impossible in the game world as DM you just say something like "this monster is unknown among civilized folk." You control the dice as DM and you simply don't offer a check (in 5e) , right? In a sense, 5e has "de-metagamed" checks b/c they are squarely in the DM's purview.Because it's mechanics. I'm totally fine with them making an investigation check and being told "the creature's skin looks unarmored and soft, and it doesn't seem that quick on their feet." But I suppose I should've stuck with the usual example of what people ask for with monster lore checks - vulnerabilities and resistances, to be clearer.
I wouldn't be okay with them making a check to know that a relatively obscure monster like an Obyrith is immune to poison. Unless the PC has something like the sage background or the ranger's favored enemy ability that would apply to the situation. There are abilities that specifically apply to this sort of thing, and lacking those, I want to maintain some degree of surprise and mystery about the world - not everything can be learned with a dice check.