A megadungeon campaign differs mechanically in several important ways from a normal campaign. This is often treated as "information everyone knows" and yet never documented (much like the actual procedure for hexcrawling).
Sessions are objective-focused not plot-focused. Each session revolves around the completion of a specific quest. Sometimes this is a request from someone with a reward granted upon successful completion, sometimes this is the players with a specific goal, such as finding the location indicated on a treasure map. The dungeon is a puzzle (or death trap) designed to be solved, rather than a story to be completed.
Movement and turns are tracked rather strictly and in a game-like fashion. Historically turn is approximately 10 minutes, there are six turns an hour. This is not rigid, a turn means “the time it takes to complete a significant action". During a character’s turn the whole party may move, or each character can engage in an individual action, such as picking a lock, try to bash open a door, looking for secret doors, etc.
Players may move a number of 10’ squares as indicated by their movement. An unencumbered 5th edition party may move 12 squares or 120’. Encumbrance slows this pace. A 5th edition party with an encumbered character can only move 8 squares. This is assuming careful, quiet, cautious observant movement. Players that move more quickly over unknown ground receive substantial penalties—always surprised, trips all traps, no mapping or distances given, hazard die rolled every turn, etc.
Traditional dungeon exploration uses an encounter die that is rolled, with a 1 indicating an encounter. More modern old-school takes on this turn this die into a “hazard” die with every result indicating some sort of decay of resources. This die is traditionally rolled every other turn, or three times in an hour. Often this die is rolled additionally in response to players arguing, making noise, or wasting time. The Hazard Die for Numenhalla is as follows.
2: Monster Sign
3: Torches Burn
4: Torches & Lanterns burn, Ongoing effects, conditions, and statuses end.
5: Rest or gain a level of exhaustion.
6: Dungeon Effect
I generally pre-generate 6 encounters or so, and select one randomly when this occurs. It is perfectly acceptable to generate encounters on the fly, which often happens when players exercise their agency to go anywhere in the dungeon they wish.
The encounter begins per the standard rules 20’—120’ (2d6x10 feet) away from the party as long as they are within detection range. If you roll 100’ for the distance, and the farthest visible point of dim light is 80’ away, start the encounter at 80’. If you roll 100’ and the party can see 60, but psionically detect opponents to 100’ then start the encounter at 100’. If either party is surprised, then the encounter distance is 10'—30’ away.
This will frequently require adjustment based on the layout of the immediate area! You are encouraged to use your judgement to create a reasonable scene based on what the dice tell you. If you are in a giant room, and are surprised by trolls, have them drop from the ceiling or climb out of a secret hatch in the floor, or burst through a nearby door moving to the encounter distance indicated by the die.
The combination of the randomness of the encounter and your skill at integrating it into the current action contributes significantly to the emergent gameplay of the megadungeon.
This is identical to an encounter roll; except the players will usually be aware of the monster somehow and the monster will be in the dungeon out of sight. Perhaps the players hear the monster or see signs of its passage. You choose a location for the monster, and when the players take a turn, the monster moves its movement in a random direction or a direction based on your judgment. They then become another entity moving around the dungeon. If the players continue to follow where it has been, then they will continue to see signs of its activity. Alternately, the players may attempt to track down the monster.
It is this cat and mouse that make the feature of empty rooms significant.
Torches are either brightly lit, dim, or burnt. Each time this result occurs, lit torches decay. Brightly lit torches provide 40’ of illumination, 20 feet of bright and 20 feet of dim.
Dim torches provide 20’ of illumination, 10 feet of bright and 10 feet of dim.
Burnt torches do not provide light.
5 torches are a significant item.
Torches and Lanterns burn
Lanterns are always brightly lit. A single flask of oil will survive 3 depletions. On the 4th depletion, the lantern goes out. A lantern is a significant item. A flask of oil is a significant item.
Some things to keep in mind regarding lanterns and torches. They take a hand to use. If holding a torch or lantern in your shield arm you cannot use your shield. If dropped, lanterns have a 2 in 6 chance of breaking and starting a small fire. If torches are dropped they become dim, dim torches that are dropped become burnt. It takes a move action to set a lantern down gently.
Also, variable effects such as nausea, paralysis, temporary blessings, or other limited conditions end when this result is rolled.
Characters must spend this turn at rest, checking their equipment, eating, catching their breath. If they do not, they gain a level of exhaustion. Adding this to the hazard die, rather than attempting to recall when 6 turns have passed makes this easier to keep track of. Unencumbered characters may ignore this result one time.
Each area in the dungeon has certain features that help distinguish it from other areas. When this is rolled, one of the listed effects occurs. This can be anything from sounds in the distance, to monsters being released, to blessings, curses, flooding, tunnel collapse, wormsign, etc.
Doors are inimical to dungeon explorers. Unless otherwise noted, doors are stuck. Most doors have a listed difficulty. If not, they have a Strength check DC of 13 + 2 * the Dungeon level to open.
On a failure, they door does not open. The players may try again, but no matter what they roll, the door won’t open.
Once open, unless a player specifies that they are holding the door open, the door rapidly shuts. Players may choose to spike a door open, but this triggers a roll of the hazard die. Unless they are one way doors, players need not check to open an already unstuck door.