Traps, Hazards and Terrain: Detection

Why call it a trap if you know where it is and what it does? If a player knows the location of a dangerous area, they will simply avoid it. Sure you could place traps precariously around a limited area, but what this does is basically communicate to the players that they simply shouldn’t move in those spaces. That’s not a trap, that’s a hazard. What’s the difference between moving into a trap’s area that can deal 2d10 damage and knock you prone and falling off of a 20 foot ledge? Not much. At least, not enough.

As I asserted in the last installment of this series all about traps, hazards and terrain, a trap should be defined by 4 things, one of which is the quality of being hidden. We have a system in this game for uncovering hidden things, and that’s called Perception. Although I feel like the passive Perception feature in 4e is a nice evolution from days of yore when there were constant Search, Spot and Listen checks, I’m not convinced it should be entirely used for detecting the placement of traps. Now before you cock an eyebrow knowing that passive Perception was developed primarily for this purpose in the first place, my argument is that it seems a trap’s entire area of effect is effectively revealed if a character in the group has a high Perception. I want that player to be rewarded for their choice to excel at Perception, but I also don’t want all the tricks out of the bag as soon as they walk in the door and combat begins. What’s the point of designing traps at that point, if you immediately reveal them when the characters gain line of sight?

I realize there are ways to manipulate this: 1/ use blocking terrain 2/ use a higher level trap, and therefore a higher detection DC 3/ just reveal what the character actually senses, not necessarily the trap’s area, etc. I’m not satisfied with these answers though. The third one however is the one that is getting closer to what I’m after. In the original Dungeon Master’s Guide traps often had multiple Perception detection DCs, revealing indications of a trap’s presence to different levels of skill. But let’s be honest, if the DM notes there are a set of holes in the ceiling above this particular set of squares, you know something fishy is there, dangerous area equals hazard, and we’re right back where we began. Again, I realize you can place the trigger in an area not directly under those holes, and that’s a nice little trick, but honestly, that requires tailoring for each encounter. I can do that on my own as well, but what I’m looking for is a system, something that works across all traps employing subterfuge.

Another thing that irks me about a simple passive Perception score revealing the location of traps on a battlefield is that with one character investing in the skill, the entire party is privy to the absolute areas of danger. I find this breaks verisimilitude and makes designing traps a waste of time. And let’s face it, because Perception is Wisdom based, a useful skill and on a lot of classes’ skill list, there’s a good chance you’ll have a good spotter in the group. And if you don’t and you’re using a lot of traps against them, shame on you for exploiting their weaknesses. I’d rather have a rule set that caters to any kind of group and rewards individual and cooperative work to overcome obstacles.

To do this I have designed a simple two-tier system of detection that works in conjunction with the rules already outlined in the source books. These rules beef up the concealment and surprise quality of traps and are about as simple and streamlined as I can make them. Here’s how it works:

Traps employing concealment have two traits: a vicinity square and an area:

Vicinity Square:

A vicinity square is a single square that indicates the surrounding or nearby location of a hidden trap. If a player beats a trap’s Detection DC with their passive Perception score, the vicinity square of the trap is revealed to them. If a player beats a traps’s Detection DC with their passive Perception by 5 or more, then the entire trap’s area is revealed to the player and the player can make an appropriate knowledge check to determine the effects of the trap.


The area of a trap can be in any shape or form, but is commonly a burst with its vicinity square at the center of the burst. The more dangerous the trap, the larger the burst or more elaborate the area is. When a creature enters the area of a trap they must make a Perception check to avoid the triggering attack. If they fail the Perception check, the trap triggers. If they succeed on their Perception check, they can move through the trap’s area without triggering it until the start of their next turn.

These rules accomplish several things:

  • Traps have the potential to surprise an adventuring party, even if the vicinity of a trap is known.
  • Players who have invested in Perception can help reveal the vicinity of traps on a battlefield.
  • Players who have invested in Perception are still more capable at avoiding traps altogether.
  • Tactics for raising the level of traps to make them more difficult to detect, as well as indicating only specific qualities of a trap to retain its intrigue still apply.
  • Characters can have no knowledge, a small notion, or full disclosure to the presence of traps on the battlefield.

llustration of the system:

One might say that this decreases the likelihood of a trap’s effect occurring, as now not only is there an attack roll, but a successful Perception check moving into the area negates any attack at all. I would pose however that this opens traps up to a more broad range of danger; you could design them to make an attack and have the Perception check for entering the area as above, or you could have the triggering attack simply have an effect of damage or imposing a condition without any attack at all. Moving into the area should be dangerous and unexpected, but spry and intuitive characters should have a decent chance of negating the effects.

Note a few things as well: 1/ When the Perception check is failed for entering the area, the entire trap triggers. This may have many forms as individualized by the trap, but typically it would attack everything within its area. This could be a deliberate choice by a character or monster to use the trap to their advantage. 2/ A trap’s area can be any manner of shape on the field, but should generally adhere to bursts, blasts and walls. If the area of a trap is still unknown, anticipation and fright can ensue. See the different arrays of vicinity squares and areas illustrated below.

Finally, I’ve provided another little trap that uses these rules so you can see it in action. The trap simply has its area and vicinity square described in the section under Traits. The Perception mechanic is reinforced under the trigger and the rest is basically set up like any other monster. I hope you can take these ideas and weave them into your own game. Until next time when I hope to take traps even a little further by exploring different ways traps can be hidden!


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