[RPG] What information does the GM provide players when dealing with a monster? What do they have to roll for

Knowledge Skill: Monster Lore

You can use this skill to identify monsters and their special powers or vulnerabilities.

Check: In general, the DC of such a check equals 10 + the monster’s CR. For common monsters, such as goblins, the DC of this check equals 5 + the monster’s CR. For particularly rare monsters, such as the tarrasque, the DC of this check equals 15 + the monster’s CR or more. A successful check allows you to remember a bit of useful information about that monster. For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.

When the players's characters meet a creature, the GM (or the players) could ask for a Knowledge skill check. Normally, people will first roll the dice and then ask what skill they should use, as it varies from monster to monster. The higher the result, the more information they recall about the creature.

The GM either has to look up the monster's stats and consider what could be useful on that situation and worth recalling for the character. Or ask the player what he would probably remember about the creature. The later is prone to metagaming, and the player could ask for things the monster might not have (a tiger has no vulnerabilities).

The GM should be careful to not give out information that is already obvious to the characters. An orc wielding a two-handed heavy axe that you identify that it is certainly a battle-axe and not a handaxe is not an useful piece of information in any form or shape, as that hardly helps against that orc.

Equipment is usually descriptive when you describe the encounter or the actions taken by the enemies. If said orc seems like he is wearing a breast plate with leather scraps, then that's probably a given that is is wearing a breastplate as his armor and will probably have at least 16 AC.

What you shouldn't give out as information:

• Any mechanical game stats;
• The previous item covers attack bonus, AC, hit points, hit die, saving throws, DC for abilities, how their special abilities work, their prepared spells, their known spells (*), their equipment that is not obvious to see, the amount of gold they are carrying in their pockets.

Let me just add that nothing stops you from giving out a creature's stats, but you don't have to, there are no rules saying you should. I know that some GM's prefer to hand out stats, but that moves the game a bit from a roleplaying game and more towards a tabletop game.

"*" If the creature has a known spell that is at will or that the creature is known for using it often, then that might be ok. Such as a blinking dog teleporting around at will or an imp that is nearly always invisible outside of combat.

What is a good narrative information that you could give them:

• The name of the creature. That should be the first thing identified.
• Rarity of the monster. Is he normally seen around there? Is he from another place? Is it normally summoned? Normally, describing the Environment information from the bestiary is enough (this is a beast normally seen on cold mountains, what brings her to this forest, nobody knows...).
• General information about the creature, this is usually the first few paragraphs of text about the creature, this might be even more informative to the characters than saying that the creature has this or that special ability.

• Gnolls are a race of hulking, humanoids that resemble hyenas in more than mere appearance; they show a striking affinity with the scavenging animals, to the point of keeping them as pets, and reflect many of the lesser creatures' behaviors. Gnolls are capable hunters, but are far happier to scavenge or steal a kill than to go out and track down prey. This laziness impels them to acquire slaves of whatever type is available, whom they force to dig warrens, gather supplies and water, and even hunt for their gnoll masters

• Make up a legend their heard about this creature before, and that's how they knew about it (The dreaded ghasts of the Moonscar River). If the character is a wizard or has invested a good amount of skill points on that knowledge skill, you could make up the name of the book their read it from (The Tale of the Green Manticore).

And there are things you could say without any knowledge check, such as:

• The header text on the bestiary, yeah, the one in italics, that is usually a good descriptive text for someone who has never seen this creature before.

• This walking corpse wears only a few soiled rags, its flesh rotting off its bones as it stumbles forward, arms outstretched..

• What weapon they are wielding, if it isn't an exotic weapon.
• What seems to be their armor, if a player asks. You don't have to say what armor it is specifically, but a general idea is enough. Leather armor, leather scraps, leather armor with bits of metal on vital areas, a huge plate of metal covering their chest, etc.
• Everything that you could tell simply by looking at an image on the creature (big teeth, long tail, green fur, glowy red eyes, four arms with claws, etc).

What is a special ability or vulnerability then?

This one is actually easy to answer. Look up a monster's stats. Read it thoroughly.

Now, what is the first thing that comes up when you think about it. That's the relevant information!

If the creature is a dragon, this is usually their dragon breath, their powerful spells, their high intelligence, or their frightening aura. If the creature is a ghoul, that should be their disease and paralyzing touch. If the creature is a succubus, that should be her charming and polymorphing powers. For a troll, that would be his regeneration that makes them immortal if fire is not used to burn the wounds, and so on.

Just keep in mind that this is an information that the character remembers. So he read or heard about this creature before, or a creature similar to the one he is looking at (or listening about). So, you have to think what about be something they would remember, what stands out most, what would make them scared of this creature.

Some facts about a medusa are completely irrelevant when you know that she can turn you into stone if you look into her eyes. Like, who cares if her bite is poisonous?, a lot of creatures are poisonous, but few of them can petrify you.

A vulnerability example is how trolls can be hurt by fire and acid.

Some creatures will have no special abilities, but their defenses or regenerative powers makes them strong. Knowing how to prevent their regeneration is something more useful to know than how much their bite hurts

Knowledge checks are not an action

This is exactly what the rules say. You don't have to waste an action to make a knowledge check, not even a free action. Which means you can make the check even when surprised and it's not your turn.

That doesn't mean you should tell the player everything about the creature before the combat even starts. Otherwise you might see some bad metagaming from the other players who just heard this information before the character that knows it even had the chance to speak.

Normally, it's good practice to wait for the character's turn before describing the result of his roll. So, after hearing what he knows about it, he can tell others on his turn using his free action to speak.