When I’m designing encounters, I often break the process into three parts, using at least two of these elements but ideally incorporating all three. They are creatures, environment, and terrain. I provide a lair, encounter, and minions suggestions for some boss creatures but using these guidelines to flesh those suggestions out. I’ll go through each definition briefly.
Creatures
The creatures include monsters which need to be defeated in some manner, but also other denizens of the area. This includes flora, fauna, hostages, guards, minions, and monsters. Beasts in the environment might be befriended and provide information to a character. The local fauna might be grown out to provide cover. A hostage might need rescuing without disturbance of the other creatures, including other innocents.
Environment
The environment is the kind of place and the general characteristics of where the encounter will take place. Environment includes the weather, air quality, wind, darkness, lighting, humidity, etc. Something like humidity might mean the environment is damp, making surfaces wet and harder (disadvantage on checks) to climb, traverse, or fight on. High winds can hinder ranged attacks giving them disadvantage while in the environment. An environment might be described as an arctic plain crisscrossed by ice and fissures in the earth. A biting wind blows incessantly, snow and ice particles stinging the eyes.
Terrain
The terrain is the physical shape of the land, building, cavern, ship or otherwise. It can be represented by a map and has dimensionality to it, even if that dimensionality is flat. It can be described in terms of walls, doors, passageways, chasms, and other spaces. It might be affected by the environment but overall is static and unchanging whereas environment might change. Descriptions which include the size and shape of an area fall under the terrain.
