(Almost) Never Zap Trap
A zap trap is any trap that deals damage to a character but doesn't provide any real means of avoiding damage. For example, if the DM decides that a tile of floor will contain a spike trap that deals 6d6 piercing damage, with a Dexterity save for half, there is no definitive way of avoiding that damage short of announcing 'I check for traps' over and over. Most often, this is a terrible way of conducting traps.
Zap traps violate that all-important element, choice, and really only serve to provide frustration when you encounter them alone. There are times when the zap trap can be appropriate, typically as a punishment for a puzzle or an obstacle to be dealt with mid-combat, abreast of typical enemies. In fact, employing obstacles in combat is so important, it deserves its own article later on. The mere philosophy that a trap needs only a damage score and a location is the true flaw here; an effective trap requires a little more thought.
Someone Has to Live Here, You Know
If you are doing traps correctly, there should be a very blurry distinction between them and puzzles. This is because, ultimately, traps are not intended to be universal killing machines. The trap's designer, or whoever happens to occupy the dungeon where the trap is laid, requires some means to disarm or bypass it, lest they become a hazard to him as well. There should always be a way around the trap. This is genuinely a story concern because it can absolutely kill immersion when your players, after being bombed for the dozenth time by zap traps, start asking "Who would ever set up these traps? No one can walk through this dungeon without being blown up!"
Make no mistake, this concern is also gameplay one. As a rule of thumb, your players should be able to out-think any challenge you throw at them, whether this is puzzles, social encounters, combat encounters, or traps. When your players feel like they have no options except to take damage from your traps, you've completely destroyed the illusion of choice and replaced it with learned helplessness. It's a terrible place to put your players.
Puzzles as Traps
To optimize for player choice and make your traps believable, their structure should be very similar to most puzzles: you present an obstacle that means the party harm or hindrance, the obstacle must contain several clues as to how it can be solved, and the players, through deduction, find one of the possible solutions. Be open to innovative solutions brought forth by your players, but know how the trap can be 'solved' before you present it.
Writing the best traps is a challenge of creativity, and placing yourself in the shoes of your players. I typically start writing traps by focusing on a mundane object with novel properties, and draw out my inspiration from there. For the example below, I'll choose a mirror, a favorite of trap-makers. Next, I construct a scenario around it that might involve a feasible physical trap or a completely magical effect. In this case, I'll use the obvious idea that a mirror can duplicate an image. Then I start to poke holes in my own, thinly-conceived device. Mirrors, for example, can't reflect things that don't leave images behind. So how does this come together?
A Practical Example: Hallway of Mirrors
A long hall lined with large, ornamental mirrors stretches out from the staircase down to this level of the dungeons. The mirrors are impressively bordered with gold and, though dozens of them line each side of the hall, only three on each side are angled to reflect a subject at a time. When a character walks down the hall and comes into the center of focus for the first six mirrors, his reflections turn to him, grinning and brandishing their weapons, striking him several times from all sides. A broken mirror releases a half-HP duplicate of the character that broke it -- destroying dozens of them is probably not an option.
By trial and error, with some deduction, players can notice that the mirrors don't attack if a character is invisible, but the mirrors also don't attack if the character in focus doesn't have a visible weapon (for the mirrors' images are formed without weapons themselves.) This presents the two most likely solutions: make the entire party invisible, or disarm them completely, storing all weapons in a bag of holding or similar location. Other solutions might be possible, like having a very weak NPC commoner smash the mirrors to make fighting duplicates rather easy indeed.
The Reverse Intuition Trap
Sometimes all you need to present a trap is to take the intuition your players might have and completely reverse it. These are my favorite types of traps, and I'm notorious with my players for using them. Always, they require lateral thinking and a healthy amount of second-guessing. Here's one example that I used in one of my campaigns, and how my players solved it:
A Practical Example: The Thief's Tamper-Proof Trap
The party is infiltrating a warehouse controlled by the Thieves' Guild in search of a nobleman held captive there. The place is on the edge of the docks and is eerily abandoned. Weapons at the ready, the party cautiously makes their way through the gaps of crates, watching for anyone that they might encounter. Then, the barbarian hears a loud click and feel the section of floor under his foot depress slightly. He freezes in place, and the rogue rushes to his aide, discovering a sophisticated explosive trap underfoot, apparently triggered by a pressure plate, much like a landmine. The DM rolls for the rogue's Dexterity check to disarm the device in secret, and it explodes.
This happens again, this time with the party fighter on the other side of the warehouse, and the rogue again rushes to disarm the device, this time expending inspiration to ensure a good check. It explodes again. The place is littered with the landmine devices and the rogue seems completely helpless to disarm them: they explode every time tries, in spite of her excellent stats. Moreover, she is running very low on HP as her failures mount.
Finally, the fighter, helmed by a somewhat impatient player, rushes forward, deciding that he could simply clear a path with his large pool of hit points. He hears a click underneath his foot. He keeps walking; nothing happens. The realization dawns on the party all at once: if stepped on, the bombs produced an intimidating audible click, but were only rigged to explode if someone tried to disarm them while the pressure plate was depressed. The thieves familiar with this warehouse simply knew not to worry about the clicking floor, safe in the knowledge that the bombs would be lethal to any unaware interlopers.
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