Your party walks into a room and the door shuts behind them. Instead of impending doom waiting for them, they instead find a long, narrow path suspended over a deep, dark pit where they cannot see the bottom. Across the narrow path on the other side of the room is a door.
This seems straightforward enough, but the group is on edge. The door behind them is now locked and too sturdy to break through, but they don’t seem to be in any immediate danger.
Finally, someone takes an additional step into the room and they hear a click followed by the whoosh of multiple swinging blades descending and arcing just over the narrow walkway to the other side of the room. How exactly will the party deal with this?
Running Swinging Blade or Pendulum Traps
Swinging blades are a classic staple of video games, movies, and fantasy settings of all types. They’re inherently cool because of their look and the danger they add to a game. Symbolically the pendulum appearance represents the passing of time and adds a feeling of mounting pressure, but mechanically the trap is all about patience, careful planning, and dexterity. While it’s easy to see how useful these traps are, how exactly are we going to use them in the game?
As opposed to other types of traps, a swinging pendulum (bladed or otherwise) moves back and forth at a set pace clearly seen by the players. Once the trap starts moving, there’s no surprise to them. You could have them start moving while your party is halfway across the platform to cause panic, but once the party knows they’re there the element of surprise is gone and you have to play out the danger element of the trap.
Because the trap is an active and moving obstacle the party has to get around it should be run as a combat encounter.
Swinging Blades as a combat encounter trap
You can actually run the swinging blades trap as a very interesting encounter with your players. Let’s start with the assumption that they cannot be easily destroyed or stopped, but leave the possibility open for your players to get creative. As the encounter starts you roll initiative for each blade that will swing back and forth. On its turn a blade only has one action it can take: move to its next position. Each blade has three positions: left, center, and right and it moves from one side to the other.
When the encounter starts each blade should be aligned to either right or left starting positions and some amount of blades should randomly skip their first round so that they are not all in the center at the same time. In the trap’s simplest form, players have to navigate the narrow path, not knowing the order in which the blades will move and having to decide when to move to the next location.
In this iteration it is possible that the encounter is just players being patient and timing each blade and moving across to the other side slowly. Only one player can fit in a space at a time, so the whole party going at once can cause problems. We would say that setting the trap up this way is too easy, but we’ve had groups run head first into obvious danger many times before.
What actually makes this trap difficult is the spacing of the blades and distance of the path. If the path is too short, a player could dash across it if there are no blades in the way, but the blades stop the player movement if they are down. If the path is too long, the trap becomes exceedingly tedious.
We recommend that you set the path just 5ft longer than your fastest player’s normal dash distance (without any special abilities). Additionally, there should be some spaces that are safe and blades do not pass through them. Players can identify these spaces by the faint groves on the path where the blades just barely scrape by each time.
If a blade moves into a space with a player it deals slashing damage as it passes and the player may attempt a Dexterity saving throw to take half instead. On a successful save the player may choose to roll 5ft forward or backwards into an open space. Players may not move into spaces that are currently occupied by blades or other players. We recommend you set the damage for the trap pretty low, as players who get in a bad position could be forced to get hit multiple times in a row.
Increasing the Difficulty of Swinging Blade Traps
If a set movement puzzle is too easy for your group, you can increase the difficulty with a few simple adjustments. One is variable swing speed for the blades. With a variable swing speed you randomly choose blades that move faster or slower. Slow blades wait a round at their peak positions before moving again. Fast blades do not stop in the middle position. Adding this feature still makes the trap a movement puzzle, but it will be harder for the players to keep track of the blades and their speeds each round.
Another way to adjust the difficulty is by adding a random element to the mix. A blade may delay itself at the peak of its arc, randomly. In this case, timing the blades is useless and players will need to figure out where they can stop along the way or take risks to get through sections that are packed too tightly.
When adding random blade swing patterns you should give your players the ability to roll a related skill check to figure out if a blade will move or not on its next turn once each round. A successful roll will lock the random choice in for the next round where an unsuccessful check will yield an uncertain result with the player knowing they can’t be confident about their guess.
Only giving one check per round to a player simulates how hard it is to watch all the blades at once and it incentivizes multiple players to go through at once, which will give the group the most information about what each blade is doing.
Lastly, you can obscure the blades’ locations when they are not in the center position by either having them swing up into darkness or in narrow slats in the walls just past the pit. When you run the encounter this way, the only blades the players ever see are the ones currently in the path and they have to keep track of the ones that aren’t visible on the playing field. This makes it a movement/memory puzzle and really increases the mental difficulty without actually making the trap any harder on its own.
Forcing Your Players to Action
You may want to set up a situation where your players can’t spend forever mapping out the blades and their positions. To do this, you simply have to add a bit of danger that encroaches on the safe space in the room each passing round. This can disappearing floor tiles, lowering spikes, or really anything.
The main purpose is to push your players to make a decision and test their information gathering in a short period of time. Each round they can only gather a bit of the information about the blades and if the spaces they can wait on the other side are limited, they need to map out a path to get everyone across before they are imperiled while still avoiding the blades themselves.
This will limit the players’ ability to spend forever on the trap, but it also gives them a few chances to get it right. So make sure you adjust this to your party and think about how your players will tackle the situation.
Use a Battle Map for this Trap
Normally we would say you can run traps using the “theater of the mind,” but that’s not advisable for this particular trap. The solution to the swinging blade problem is only apparent with information. When you use a battle map you give your players a way to count blades without asking you, see where blades are (or which blades are in the path), and can indicate where parts of the path are safe to stop. Without a battle map this puzzle becomes both hard to describe and even harder to solve, largely becoming a slog for everyone.
Avoiding the Pit and Not Killing Your Players
The pit is there for show, and you shouldn’t knock your players into it if you can help it. If they do fall in, climbing out would be challenge but not impossible. We always recommend you avoid killing your players outright with traps. Instead, just wear down their health. We recommend you set the DC of a Dexterity save for anyone hit by the blades depending on your party, and also recommend keeping it on the easier side.
Player Solutions You Should Allow
Walking through the blades is the trap. It’s obvious and dangerous. However, there are alternate solutions that players can achieve and you should allow them to act creatively. If players offer solutions that might stop the blades, let them try it, but stopping them by force should be difficult or require the expenditure of resources.
One solution we want to highlight is hanging on the edge of the path and shimmying across. This is a little silly, but effective, and it takes both time and energy to do. If you want to make it hard, have them do Strength checks to hold on as they shimmy across. A fighter or barbarian should easily be able to handle this, but a wizard will have a heck of time.
Another potentially safe solution is to lower a rope into the pit, walk across, and then set up a grappling hook to climb out the other side. This solution is also very silly as it avoids the trap altogether, but they really don’t know what’s waiting in the pit below either, so it doesn’t have to be a cake walk if you want to ramp up the struggle.
Really, we encourage you to let your players get creative. They might want to try all sorts of things to get by these traps and it’s more fun to let your players attempt things than to just force them to deal with the trap the normal way.
A Mechanical Trap that Runs like Combat
Not every trap is about surprise. The swinging blades illustrate that some traps are traps just by their nature and are designed to pose as an obvious obstacle. Swinging blades are a great trap for parties that look for puzzles, action, and movement based obstacles. Give them a try with your group and see what the experience of active movement traps are like for your game.