Dungeons are dangerous ecosystems and living environments. In small dungeons, characters quickly explore and leave before the environment can adapt. But in a massive, sprawling, mega-dungeon like I am writing, the timeline changes. Characters can be traversing the same corridors for months as they trek into its depths and return to safety. This extended timetable allows new threats and advantages to appear on every dungeon level.
Over time, new creatures move into the dungeon’s vacant rooms and corridors. Some will become wandering encounters, but others will make lairs. Once safe areas, can become dangerous again. As news of the dungeon attracts patrons to aid the characters, it can also lure intelligent, powerful evil things to its depths for their own sinister deeds. Gygax mentions this in the 1e Dungeon Master’s Guide (page 90), and I have applied this to my design.
Living things will grow. Patches of mold, moss, or mildew that were avoidable when the characters first entered the dungeon could expand into much larger areas. They might fill corridors, block travel, gain hit dice, and/or damage potential. Colonies of harmless rats could become swarms of ravenous ones. Special plants, harvested for alchemy or poison-making, will grow back over time. These areas might warrant a notation on the map.
The dungeon’s lurking dangers are indiscriminate. They devour anything that wanders too close. But humanoids may parlay–until they don’t. Some humanoids will parlay with characters and offer quests to aid their own intents and survival. Rewards could be gold or information on other enemies or villains. As characters encounter various humanoid factions, their interactions with each group can have an effect on the whole. Slaying goblins on sight will make other goblins in that faction exchange parlay for aggression. Or, when forced to parlay, it will only be treasure for their lives.
Intelligent monsters plan around the characters. Similar to goblins, intelligent creatures that encounter the players and survive will plan defenses and strategies for future engagements. They will utilize the dungeon they know. Over time, they will set traps, station guards, and build fortifications. Time becomes a benefit for the characters’ enemies.
Secrets can be revealed. As the secret door on Lonely Mountain could only be discovered at a certain time, so too are there places in the dungeon that are hidden until certain calendar days. Resourceful characters might learn when and where a door will be, but be forced to wait until late December to open it. These hidden areas may hold bounties of treasure or knowledge, but they can only be found when time passes.
For my sanity, events are static until they are discovered. The passing of time has no effect on the dungeon’s denizens until the characters experience the initial encounter. New monsters won’t move into the dungeon until the others are removed. Intelligent monsters won’t plan for the characters until they encounter them the first time. Players can experience the dungeon as it was initially designed before it evolves into something far more sinister and dangerous.