[RPG] How to model sinkholes collapsing under the PCs


I found the rules for quicksand but those aren't exactly what I'm seeking. While I am using a marshy environment I want to create terrain which can suddenly give way under the PCs. There are a few criteria I'm seeking from an answer:

  1. Random surface area. It's not a man-made dungeon here but a vast landscape with variable-sized potentially collapsing floors.
  2. Random height. I don't have a clear distance to drop (again, not a man-made dungeon) and would rather not model each individual drop after the same pit trap.
  3. How to determine (for game hazard purposes) when/whether one will open or not? "More weight" is really vague – is there a way to model an increased likelihood? The terrain in question is incredibly volatile so such things would be very likely… What are things the PCs can do to make it more so? Landing after jumping, riding horses, wearing heavy armor, enlarge person are all I could think of.
  4. Would it be possible (again, game purposes) for characters to use survival to tell if certain ground would give way? Not for the presence of sinkholes – I'm planning on making that horribly obvious. I mean "should I approach that treasure or not?"
  5. Would characters get a reflex save? Should this depend on what they were doing and/or the width of the opening?

The area I've got portioned off for sinkhole-intensive terrain is a good distance and not easily circumvented without magic. My party will be starting at 2nd level and if they follow my clues will choose to avoid this environment completely, for a while at least. How can I model terrain so that if they decide to challenge nature, she will most likely swallow them whole? Again – not quicksand, but sinkholes. Cave-ins that happen due to cavities below the surface, where the ceiling/floor becomes too thin to support the weight above it.

EDIT: The above model is lacking a few things which I am seeking here.

  • How to determine the formula for 1 and 2. Any additional PC-related
    modifiers that should normally be considered for 3.
  • DCs for survival, or any other related skill, and times when a check should be called for. Before walking on? While standing upon? After falling? (kidding on the last one)
  • DCs for a reflex save or other relevant check to avoid. Modifiers as well, for the variable width/actions involved.

It just hit me that the encounter level might matter for these. I'm planning on having three difficulties, one set for fourth level characters (the warning), one set for ninth level characters (worse) and one set for fourteenth level characters (nigh-certainty). I plan on barring flight through wind conditions.

Best Answer

I'm sure this won't be the most satisfactory answer, but I would be tempted to approach this problem using my percent dice. Really, the trick is just in picking a suitable "percent chance" that each factor might occur. For example, in response to your points, I came up with something like this:

  1. For every 15 feet travelled, roll the percent dice. The ground has a 35% chance (or adapt as you see fit) of giving way to a sinkhole. I'm not a strong statistician, but as I see it, the percentage you select will reflect the percent surface area of your terrain that is covered with sink holes. You can determine the chance depending on how much of the terrain you want to have sinkholes.

  2. If the ground collapses, roll 1d4 and multiply by 10 feet to get your depth. Once again, choose the multiplier based on how deep and damaging you want the sinkholes to be. Since characters can ignore the first 10 feet with a successful tumble check, and we want the fall to be damaging enough to be a deterrent (but not deeper than the average length of rope) I created rather cavernous sinkholes. You could reduce the multiplier if you find this too epic.

  3. I would adapt the percent chance in (1) to reflect the differing stress the PCs are putting on the terrain due to weight/movement, etc. Make the chance 80% for extremely large or heavy PCs.

  4. With a successful survival check (DC=12) before a taking a declared route, let the PCs see your percent dice roll from #1(but don't inform them of the DC/percent threshold for "collapse") so that they can judge whether or not they think the ground will give way.

  5. Give the PCs a reflex save (DC=12), but make them check each 5' square in their "retreat" to ensure they are not part of the sinkhole until they reach a "safe" square.

You can of course modulate the percent chances to make it harder or easier to navigate this terrain, but this should enable you to create terrain on the fly.

I don't know if I would modulate anything other than (1) based on encounter level (unless the PCs have taken on more equipment, making them heavier), as the physics of the situation shouldn't change in response to the PC level, but you can make the terrain progressively more sinkhole-ridden, the farther from the path they wander. It is also possible to start adding in creatures of the deep in some of the larger sinkholes to provide an extra challenge.

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