Combining and Separating Encounters
Source GM Core pg. 79
Picture this: the PCs storm a fortified outpost. They choose to eschew stealth in favor of a direct approach. A drone spots them and raises an alarm. The sound of alarms blares throughout the base as enemy troops ensure that everyone is ready for a fight. And then, they politely wait in whatever room they were already standing in for the PCs to come and attack them. It sounds pretty unrealistic, and it feels unrealistic at the table. Many players find it far more satisfying when their foes take reasonable actions and countermeasures against them, such as moving to defensible positions behind cover or banding together with allies. Taken to an extreme, combining encounters can quickly lead to fights that are unwinnable, so be careful. In the outpost example, some troops might come out to attack the PCs, while others cluster around the control station. Perhaps each individual patrol of soldiers around the castle is a trivial-threat encounter, but as they gather together, they form groups of gradually escalating threat. Such groups give the PCs a sense of how challenging their opposition is, so that if a fight against six soldiers is a challenge, they won't try to pick a fight with 30. When the PCs' foes amass into an overwhelming force, give the PCs fair warning and a chance to retreat and try again another day. Of course, if the PCs come back after the alarm has been raised, the soldiers are likely to change their rotations to better secure the base.
The most common reason to separate an encounter into multiple pieces is to set up a combined encounter, like when an injured foe retreats to gather reinforcements. This provides the PCs with a choice: do they ignore the fleeing enemy and focus on the battle in front of them, or do they split their own forces, weighing the risk of being led into a dangerous encounter against the chance of stopping later foes from preparing for their approach? An encounter might also separate into pieces because of dramatic changes to the battlefield, such as a collapsing ceiling or a magical wall that prevents those on each side of the barrier from accessing the other without spending actions to bypass the obstruction.
The most common reason to separate an encounter into multiple pieces is to set up a combined encounter, like when an injured foe retreats to gather reinforcements. This provides the PCs with a choice: do they ignore the fleeing enemy and focus on the battle in front of them, or do they split their own forces, weighing the risk of being led into a dangerous encounter against the chance of stopping later foes from preparing for their approach? An encounter might also separate into pieces because of dramatic changes to the battlefield, such as a collapsing ceiling or a magical wall that prevents those on each side of the barrier from accessing the other without spending actions to bypass the obstruction.