I think there’s allowable degrees, and that it’s table-dependant.
In general, knowing trolls are vulnerable to fire is fairly common player knowledge. I’ll also point out that even in The Hobbit, when the trolls petrified in the sunlight, the narrator says “for trolls, as you probably already know, must be underground before dawn.” This troll vulnerability is common knowledge in middle earth!
I think that if a GM wants a little known vulnerability, they can do a little extra work to make that easier for the players to respond appropriately to. Trolls work far better as a fairly tough monster with a fairly well known vulnerability. If you want that to be different, I’d use a troll variant, and make it clear that these creatures don’t fear fire!
I think there’s allowable degrees, and that it’s table-dependant.
In general, knowing trolls are vulnerable to fire is fairly common player knowledge. I’ll also point out that even in The Hobbit, when the trolls petrified in the sunlight, the narrator says “for trolls, as you probably already know, must be underground before dawn.” This troll vulnerability is common knowledge in middle earth!
I think that if a GM wants a little known vulnerability, they can do a little extra work to make that easier for the players to respond appropriately to. Trolls work far better as a fairly tough monster with a fairly well known vulnerability. If you want that to be different, I’d use a troll variant, and make it clear that these creatures don’t fear fire!
The DM always has the right to throw curveballs at the players, and screwing with player precconception is part of what makes dming fun.
The player is responsible for figuring out how the setting is different from expectations, and plan accordingly.