DMing Encounters (DMG)

Encounter tables are created before play begins. During a game session, the DM has to take the information he has put into the encounter tables and bring it to life.
To use an encounter table and run an encounter, the DM needs to know several things: How often should he check for encounters? What is encountered? How many creatures are there? How far away are they? Did they surprise, or were they surprised by, the characters? What will the encountered group do? The rules below tell you how to answer these questions.
==Encounter Checks==
The DM knows when a planned encounter is to occur, based on the conditions or location he has prepared. The same is not true of random encounters. For these, the DM must make encounter checks.
Frequency of Encounter Checks: How often the DM makes encounter checks depends on the situation. Different types of terrain (or dungeons) may make checks more or less frequent. Furthermore, the type of terrain and population density will affect the chance the characters have a meaningful encounter. Table 56 lists both the frequency of checks and the chance that an encounter will occur for the most common wilderness situations. If characters are adventuring in other types of terrain, the DM can use a comparable entry from the table or can determine frequency and chance of encounter himself.
Encounter Chance: This lists the number or less that must be rolled on 1d10 for an encounter to occur.
Time of Day: If an x appears under a specific time of day, an encounter check should be made. This does not ensure an encounter, it only requires the check for one.
The chance of having an encounter can be modified by several factors. Foremost of these is population density. The chances of an encounter listed on Table 56 assume an unpopulated wilderness area.
Wilderness Checks: If the region is patrolled or sparsely settled, the chance of an encounter increases by one. In heavily populated areas, the chance of an encounter increases by two. These modifiers should not be used unless the DM has specially prepared encounter tables to reflect the differences between settled lands and wilderness, however.
The DM can also choose to modify the chance of an encounter for any other reason he feels is justified. If the characters have been making excessive noise or if the village alarm has been sounded, the DM can increase the chance of an encounter. The DM can even decide arbitrarily that an encounter will occur, although it can hardly be considered random any more.
Dungeon Checks: Encounter checks in the dungeon are not affected by terrain (since there isn't really any terrain to consider). Normally, one encounter check is made every hour, with an encounter occurring on a roll of 1 on 1d10.
If the DM deems part of a dungeon particularly dangerous, the number of checks can be increased to once per turn (10 minutes of game time). The DM can also increase the chance of an encounter occurring. If the characters engage in an activity that makes excessive noise (hammering spikes or taking part in a loud battle), an encounter check should be made immediately.
==Is This Encounter Necessary?==
Any time the DM feels his adventure is dragging along or that characters are getting over-confident, he can declare a random encounter. Likewise if he feels that a random encounter would hurt the adventure, he can ignore one that's called for. Good judgment and story considerations are more important than slavish devotion to procedure.
==Encounter Size==
If the DM decides that yes, this encounter should happen, he determines how many creatures or NPCs appear. There is no quick and easy formula for this. Experience is the best guide. The Monstrous Compendium lists a typical encounter size for each monster. Use this as a guideline, especially when you're first starting out as a DM, but don't follow this inflexibly.
When uncertain, use a small encounter. It is far better for a random encounter to be easily defeated by the player characters than it is for the monster to overwhelm them. An easy PC victory gives the DM information and experience (so he'll know to increase the difficulty of the next encounter) without harming the player characters and his campaign. A crushing PC defeat is almost impossible to correct without obvious manipulation once the encounter has begun.
As always, use common sense when determining how big an encounter is. Nature provides some guidelines. Bear these in mind when figuring encounter size.
Many predators, especially those that hunt by night, are solitary creatures. A nocturnal fantasy creature might show up alone, as well.
Of the predators that hunt by daylight, some work alone while others cooperate in groups of two or three. One or two will attack the prey from one direction while the others wait for it to be flushed toward them. Such hunters are usually stronger and faster than their prey. Again, fantasy creatures can follow this pattern.
Smaller predators sometimes hunt in packs of 5 to 12, attempting to surround and harry a chosen victim. Herbivorous animals tend to flavor herds and the company of others. Omnivores live in smaller groups and often have older members that act as guards. All of these factors can play a part in the size of a given encounter.