Not all monsters are very strong and sometimes they know when they’re outmatched. In these cases, it makes sense that monsters might flee from your players in combat. This does not mean that the running monsters are dumb or defenseless. Many weak but intelligent creatures can set traps. These creatures usually use collapsing traps as a means to escape and want their predators to chase them.
When a monster is weak and runs to safety, it’s usually running somewhere the players cannot follow. The monster is trying to increase its chances of survival while also decreasing the chances it will be chased again.
Smaller monsters will use their size to their advantage. A small monster can fit between crags and down corridors that larger creatures cannot follow. If they attempt to, they often get stuck, hit walls, or move much more slowly then their prey. Creatures like kobolds, goblins, or fey creatures can use this to their advantage when setting up traps.
The Basic Collapsing Trap
If your players see a creature run through a small opening and try to squeeze after them, there’s a chance that the opening itself is the trigger for the trap. When they inevitably get stuck or push against the walls to get through, the pathway they are passing through looses support and collapses on top of them, dealing damage, trapping, and potentially suffocating them.
These traps are extremely dangerous and typically will kill lone adventurers who don’t have the help of their friends. The way these traps work is that in the pathway the monster has step supports to hold the unstable earth up. These supports are just barely strong enough to keep the path open. If something has to squeeze through the tunnel, the opening will collapse inward and crush them.
These traps aren’t complex. They often aren’t even well hidden. They look so much like normal supports that players might not know or realize that they are traps. But in less stressful situations, they might not even want to try and squeeze through such tight spaces. The traps work best in situations where the monsters who built them is the bait. The goal is to get the players to chase them somewhere they might not normally follow.
More Advanced Collapsing Traps
Sometimes the basic collapsible tunnel isn’t all that the monsters will set up. In situations where they have lairs or nests, they might go as far to have whole rooms that collapse. These kind of traps are more complex, more dangerous, and a lot more obvious.
An example of a fully collapsible room might be one where each exit has supports that the monsters can pull down on their way out. This causes the beams holding up the ceiling to give way. Another form of this might be a central column in the room that can easily be taken down, usually by a rope tied around the middle of it. The rope can either be pulled by an escaping monster or set taught as a hindrance to pursuers.
When you make collapsing traps like this, you have a really high chance of a TPK. It’s not only that the trap itself does damage, it’s just that there is little chance of escape once it’s triggered. If the party doesn’t have a reasonable way of escaping, like by using spells like Teleport, Dimension Door, or Stone Shape, they might be stuck for good.
Collapsing Traps by the Numbers
So you’ve set up one of these traps. How much damage do they do? That depends entirely on the situation.
If someone is hit by the equivalent of a falling rock, they’d take 1d6 damage and likely be trapped in the tunnel until they can be dug out.
If they are further along in the tunnel when it collapses, they’re up to 1d10 damage, the equivalent of being crushed by a full bookcase.
If they’re in a small cavern room that collapses, they would take the same damage as a collapsing roof or ceiling, 4d10, which is much more serious.
If a whole large room collapses, the damage would begin to scale up from there. Six, eight, or even ten d10 worth of damage could come down on them in even a moderately sized cave in.
If you want to get more specific with the damage for your collapsing trap, you can look in the Dungeon Master’s Guide for the section on Improvising Damage. It’s what we used to estimate these numbers.
Remember when using these traps that they’re dangerous despite being simple. Like other traps we’ve covered here, be sure to give your players an indication of the danger and let them decide if they’re going to take the risks.
Have fun crushing your players and happy DMing!