I was recently running a game with my players, whose characters were in combat with a Scarecrow. According to the Monster Manual, Scarecrows have immunity to poison damage, but my players weren't rolling checks to gain information about the creature. When one of my players attacked it with their poisoned weapon, I was stuck wondering how to proceed.
When player characters' attacks fail because of immunities or resistances (or do double damage thanks to vulnerabilities), is it a best practice to flat out tell them about the resistances (etc.), or hide the results and pretend they work?
Best Answer
"Your attack appears to inflict less damage than it should"
is the generic response my old DM preferred to use, very regularly. I try to storytell it a little better, saying how it "didn't slow down the creature at all", or "it batted away that attack". "It didn't flinch at the blow", or "the majority of the poison/acid/fire/whatever seems to slide off the monster's flesh".
Generally, if you imagine what a visual effect a poison effect would have (sickly green ooze, maybe?) would have in an attack, and what would describe it being fairly useless, you can come up with something on the fly. It does get a little more difficult with complex damage types like psychic, force, or poison, but simply saying "it doesn't slow down or flinch, or even notice" is a great way of telling your players what's up.
This could also be used for temporary hitpoints, which is a good way of keeping them on their toes, so you aren't giving away all the possibilities by dropping a few hints that something's wrong. "Something's off" could be temporary hitpoints, an illusion, resistance/immunity, or a disadvantage roll you might have rolled in secret (in case they had a hidden ability that caused a player to attack them with disadvantage, like Phasebeasts)