"Placed" are those you know are in a particular place in the dungeon. Whether a random roll told you they were there or you decided to put them, they are certain to be in a location.
"Wandering" monsters may appear, especially when noise is made, anywhere in a dungeon and at any time, if a roll says they do. There is a certain line-up of possible creatures which might appear, but it is never certain they will, or which of them will. Players could clear a whole dungeon and never encounter one of these.
"Discretionary" monsters appear when I, the DM, decide they do. There are most likely one or two I've prepared ahead of time for any dungeon. The monster comes and goes when I decide it does.
Now, hold on, you might be thinking, how is this old school? It may seem a bit hypocritical for someone who believes so much that "story" is what emerges from play, and all the random rolls that move play forward. But my game is fairly low magic, where you are likely to encounter wolves, and bears, and difficult terrain. I like my game that way. An adventure game that is so magical that wolves in a forest would no longer be considered a threat, because there are manticores, and minotaurs, and dragons in every hex, would lose much of what is thrilling about adventure tales from the real world.
And yet, I'm not trying to simulate frontier living, I want that spookiness of fairy tales. If all that ever show up in my dungeons are wolves and bears and I have no control over when they appear, a dungeon might become a very mundane affair. I want players that go underground to be afraid. And to fear more than the loss of their lives. And so I started keeping some monsters on "reserve." They would show up at times players felt most vulnerable. And it really worked. It freaks them out.
Now, to avoid my becoming some adversarial DM that kills parties by making monsters show up and attack at my whim, I make these discretionary monsters non-violent. They don't attack, even magically. What kind of monster you might be thinking? Well, old ghost story standards: a lost child, who by all rights shouldn't be down in the dungeon, who says something before slinking off into darkness. A talking animal. A little man riding a dog. Something odd, but not outright threatening.
You might think that players will, after a while of seeing many of these, catch on to these non-violent monsters and no longer fear them. But it turns out that it can be difficult to tell between one of these and a wandering monster, or if the characters have just entered a room, a placed monster. In other words, it is never quite clear to them what type of encounter they are having so as long as a dungeon has all three types it will always be in a player's best interest to be wary, if not fearful.
|"Might I hold your sword for just a moment?"|
- Appear when players feel vulnerable. See here for some thinking on that. I find when characters are climbing up or down long drops a perfect place, either the first person to climb down a rope is pulled aside and told about what they see, or the last person to descend is told something is stirring in the shadows.
- Are hard to determine if they are a threat. Something odd or off about a normal thing is pretty much the definition of creepiness. The "why are you down in a dungeon, child!?" is probably the perfect example. But a cat that comes out, sits, and offers a hand to shake, or a dog that comes out and starts coughing as if to vomit could work.
- Not outright hostile. I think a Blind Agnes is an example of a creature that, while not a pack of orcs with swords, would cross the line for me and be unfair for me to use when I choose. It would steal the vision of a player, and while creepy that is an outright attack. And yet, these types of monsters don't have to be harmless, which leads to the next point:
- Will make the situation even more dangerous if you engage with them. The monster seems to want to lead you somewhere else. The Blood Dove and Greater-Crested Potionguide would both work well as discretionary monsters. But children or peasants might ask characters to do something that would be dangerous to do: "Close your eyes," "Hand me your sword for just a moment," "Follow me, I have something to show you." I don't think it matters that few players would be foolish enough to do these things, even having the option presented to you is creepy.
- Used sparingly. This gives me the option to do something a computer can't do, make monsters appear based on what I can tell about player mood. Good times to deploy are when players are getting bored, distracted, or play is starting to slow down into a slog. These kinds of monsters allow me to put a spark back into the game. But they work because they create tension, and if they appear around every corner there is no tension. Once or twice a dungeon is usually plenty.
- Aimed at particular players. This is another thing that a computer or random system would find difficult to do. I like for new players or quiet players to see these. Then, it gives them something they can, and even need to tell the rest of the party, it gives them a reason to interact with the other players. I like pulling them outside the room to tell them what they see because it causes tension with the other players, the rest of the party will really want to know, now, what that player was told. It makes the new/quiet player the center of attention for a bit. Note, that this all only works because these discretionary monsters are non-violent. When a wandering monster shows up it is usually quickly evident to everyone.
*Yeah, it's an accurate but horrible name. If you have an idea for a better one let me know.