That's only half the story. For the DM, wandering monsters are like the props they hand out at improv. As Chgowiz shows, they are spurs for your imagination, the seeds of new angles on the plot and fresh adventures. Most importantly, when the players zig when you expected them to zag, wandering monsters give you something to fill in the blanks. What's in that unmapped hex? The dice say: bandits. Are they a wandering band that infiltrates pilgrim caravans like the Thuggee Cult of India? Or are they unemployed mercenaries who have turned brigand? Maybe they're a military force from a neighboring kingdom come to infiltrate in preparation for... what? Invasion? Assassination? Cattle raiding? Or are they terrorizing simple peasant farmers like in “The Seven Samurai”?
The lists in the rulebooks are fine starting points, but you're probably going to want to make your own. If you do, don't feel constrained by the formats you've seen, especially if the whole improv idea leaves you cold. There's no reason you can't put more detail in your wandering monster charts. A group of six orcs might be the hapless minions of the evil sorcerer, busy screwing up yet another simple task given to them, or they might be a hunting party returning with fresh game for the stewpot, or gamblers looking for a quiet place to roll the bones and win or lose a few coin. In the wandering monster tables from D3: Vault of the Drow, Gygax tells us:
- the bugbears are “going about the businesses of one of the merchant clans, and they will bear a distinctive broach”
- pack lizards are docile grazers of edible fungus, if left alone
- trolls are “employed by the Drow to maintain discipline amongst their other servants”
- the edible fungus “ripens rapidly, and crews of workers must harvest the stuff for food (the tough outer skin being used for many other purposes)”.
If you need to define just why those orcs are wandering through when they bump into the PCs, go ahead. If you're fine with just jotting down “some sorta monster with 3 HD that flies”, that's cool too. These are your charts for you to use when you're playing, so do whatever's most helpful for you.
It's probably a good idea to weight your tables towards certain encounters. You can do this either by spreading single sort of encounter across multiple numbers (the orc hunters are encountered on a roll of 2,3, or 4 on a d8) or by using probability curves (the orc hunters are encountered on a roll of 10 or 11 on 3d6). That way your wandering monsters are more likely to express the makeup of your dungeon, and your players can use that information to guess something about the sorts of creatures they're likely to encounter.
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