Random Encounters (DMG)

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In addition to planned encounters, the DM also runs random encounters. These aren't tied to a specific place or event. They are based on chance.
During the course of an adventure, the DM makes encounter checks, rolling a die to determine if a random encounter occurs. If one does, the DM chooses or randomly rolls for an encounter using a random encounter table he has prepared or one provided with a published adventure. Complete random encounter tables are provided in the Monstrous Compendium. These can be updated and replaced as new creatures are added to the 2nd Edition AD&D game.
When a check indicates an encounter is imminent, a creature or NPC determined by the encounter tables will arrive in the area in the next few minutes to investigate. Many encounters end in combat, but this isn't necessary—it is possible to talk to intelligent creatures, whether in the dungeon, out in the wilderness, or on the streets of a town or city.
==Should You Use Random Encounters?==
Some argue that random encounters are foolish and should not be used. These people maintain that everything should be under the control of the DM, that there should be no surprises for him while playing the game.
Certainly, random encounters can be abused through overuse, and they can create illogical encounters. (The word foolish would certainly apply to the DM who allowed the characters to be attacked by an orc war-band in the middle of a peaceful human city!) However, when used judiciously, random encounters add to everyone's fun in a couple of ways.
===Variety===
Random encounters introduce variety the player characters didn't expect. The characters, exploring a dungeon, become overconfident if they only encounter monsters in chambers and rooms. Random encounters reminds them that any second could be dangerous, no matter where they are.
===DM Challenge===
Random encounters make the game more exciting for the DM. The game has to be fun and challenging for him as well as the players. Part of the challenge for the DM is to improvise an encounter on the spot. The DM gets involved and excited, improving the play of the game.
To use a random encounter, the DM doesn't just open his rulebook and blindly pick a monster (although there is nothing that says he can't do this). Instead, he uses or creates specific tables that are tailored to the needs of the adventure and adventuring area, by including only those monsters or NPCs that are appropriate to the setting.
==Characteristics of Random Encounter Tables==
Characteristics of Random Encounter Tables
All encounter tables share certain concepts. Before you begin creating your own tables, some understanding of these basics is necessary.
===Uniqueness===
Although one could create a single encounter table and use it for every situation, this is a grievous limitation on the wealth and detail possible in a campaign world. Encounter tables add distinction and differentiation to areas. Encounter tables can reflect conditions as basic as terrain or as complicated as entire social structures.
This in mind, the DM should decide where in the campaign world each encounter table applies. A single table could be made for all deserts; a separate table could be made for the Desert of Shaar, which is noted for its fabulous beasts; a further table could be made for the ten-mile area around the Palace of Yasath in the Desert of Shaar, where the Emir of Yasath maintains patrols to keep the beasts at bay. Within the palace an entirely different encounter table would be needed, since the patrols don't tramp through the hallways and harems.
Each table says something about the conditions in a particular area—the level of civilization, the degree of danger, even the magical weirdness of the area. Although the players never see the entire table, such tables help the DM define for himself the nature of his campaign world.
===Frequency===
All monsters have a frequency of appearance, whether given in the monster's description or assumed by the DM. Orcs are more common than minotaurs, which are seen more often than dragons, which, in turn, are seen more often than Tiamat, Evil Queen of the Dragons. Frequency of appearance is normally listed as common, uncommon, rare, very rare, and unique.
Common creatures normally account for 70% of the local population. They may be more prolific or just more outgoing, more likely to show themselves to strangers.
Uncommon monsters fill the next 20%. They are fewer in number and tend to be more wary of outsiders.
Rare creatures account for another 7%. Such creatures are normally solitary, exceptionally powerful, or very retiring.
Very rare creatures constitute only 3% of the population. They are truly exotic and almost always extremely powerful. They may be creatures who have wandered far from their normal range or whose magical nature is such that not many can possibly exist at any one time in any one place.
Unique monsters are just that. They are individuals, specific and named. Such creatures should never be used on random encounter tables. They are reserved for planned encounters.
The chance of encounter is not determined solely by the frequency listing, however. The DM should also take into account a location's terrain or deadliness. A polar bear can be considered unique only in the tropics and is very rare at best even in the northernmost reaches of temperate lands. An orc living in the deadliest area of an ancient ruin, an area populated by a dragon, mind flayers, and medusae, would be very rare indeed (and very lucky to be alive). Frequency must be modified to suit conditions.
Frequency must also be subservient to the conditions the DM desires to create. If the DM wants a valley filled with magical creatures of incredible deadliness, then rare and very rare creatures are going to be more frequent. A lost valley filled with dinosaurs defies the normal chances of encountering such beasts. Indeed, they could only be considered unique elsewhere.
Furthermore, frequency does not mean characters will encounter a creature 70% or 20% of the time, only that it falls into a group that composes that percentage of the population. The percentages and ratings given are not demographic data; they are only guidelines.
Several common creatures will compose the bulk of the population, so that the chance of meeting any particular type is less than 70%. The same is true for all the other categories. In the end, the chance of meeting a particular type of common creature is still greater than that of meeting an uncommon or very rare creature.
===Logic===
The other significant factor restricting encounter tables is rationality. Everything on the encounter table should be justifiable for one reason or another. By requiring justification, the DM can quickly narrow his range of creature choices down to a reasonable number, in essence winnowing the chaff from the wheat.
The first and easiest criteria are terrain and temperature. Camels aren't found in jungles: kraken don't crawl across deserts. Glaring contradictions of logic must be justified. Produce a woodland dryad in the middle of a barren waste and the players are going to demand some explanation. Worse yet, they may assume the encounter is significant to the adventure because it is so illogical, which may in turn throw your entire adventure off track.
Even if the creature fits a given terrain, it may not be appropriate to the setting. Just because an orc can appear on the plains doesn't mean it should, not if those plains are at the heart of a fiercely guarded human empire. Out on the fringes where raiding bands could slip across the border would be a far more appropriate place.
As important as terrain and temperature in assessing the logic of a random encounter is the character of the society the table is supposed to reflect. Balance what the players expect to meet with what would make a good adventure. At the heart of an empire, the characters would expect to find farmers, merchants, nobles, priests, and the like. The task for the DM is to find ways to make these seemingly ordinary encounters interesting.
In wilderness areas and abandoned ruins, there may not be a particular culture to consider. However, there is a society of sorts or, more accurately, an ecosystem. This is often overlooked in dungeon settings. Just which creatures feed on which? What relationships exist that allow all manner of diverse creatures to live in the same place without annihilating each other? Does a creature's random appearance make sense with what the characters know about the place? Medusae make poor wandering monsters, since logic says there should be statues of their victims in areas where they live. To round a corner and run into a medusa who just happens to be strolling the caverns grates against logic.
===Effect===
Finally, as DM, consider the role of the random encounter. Such an encounter is not a part of the adventure being told; it hasn't been worked into the plot and doesn't advance the conflicts. A random encounter should not be the most exciting event of an adventure. You don't want the players remembering only the random encounter and forgetting the story you worked to create!
Random encounters provide breaks in the action and can build or release tension. The characters are galloping after the desperately fleeing kidnappers. Suddenly a flight of griffins, attracted by the clamor of the chase, swoop down, aiming to make a meal of the player characters' horses. The kidnappers may escape unless the characters can extricate themselves from the attack in mere moments! The tension level goes up.
Random encounters can also wear the player characters down in preparation for a larger, planned encounter. The uncertainty of the encounters adds an element of risk for the players. Will the characters be strong enough? A random encounter should rarely cripple a party (unless they are in a sorry state to begin with), but each one should weaken them a little.
It doesn't matter if the player characters win every random encounter, especially not if they are down a few more hit points, spells, and magical items after each. Just knowing they are not at peak form and that they have expended their abilities on wandering monsters makes the players nervous.
For these reasons, you don't want to use the most powerful and significant creatures when creating random encounter tables. You certainly don't want to use creatures that are more powerful than those in the rest of your adventure! Random monsters should be less significant than those you have planned.