Trap Showcase: A Lesson in Chemistry
This is a great example of how real life can inform our hobby, in this case, how the effects of real-life chemistry can be used to create a uniquely unexpected yet devastating trap that plays with the expectations of your players.
So, what kind of chemistry are we dealing with here? None other than electrolysis. No, not the hair-removal kind; that’s a different handbook altogether. In this case, I’m speaking of electrolysis in terms of sending a direct current through water, which splits H2O into hydrogen and oxygen gases. Hydrogen is remarkably flammable, and the resulting mixture can be highly explosive. You wouldn’t want so much as a bit of metal to strike the ground the wrong way around such a combination, let alone to have an open flame as a massive cloud of fuel rises into the air.
That’s where the concept gets fun: you get to describe the trap in a very innocuous, possibly even idyllic, way. The setup is simple, because all you really need is water. One of the best ways to provide it is a walkway flanked by two channels of flowing water, but it could come from any number of sources.
The real trick is how you plan on providing the electricity. Technically speaking, producing a notable amount of gas would require a continuous flow of direct current through the water, so if you want the setup to feel “realistic,” I would use some kind of battery, mechanical or magical, which sends a current through the water. Alternatively, you can go for something more dynamic, such as a swarm of shock lizards serving as a constant source of electricity.
Once the cloud of explosive gas is in the air, you’re free to trigger the actual trap with basically anything that creates a spark. Your party is holding a torch in a dark, dark dungeon corridor? Well, now they’re holding the trigger for the explosion. The barbarian swings an axe into a foe wearing plate armor? Now they’re both going to be extra crispy.
What I love about the trap is that it really doesn’t matter what the source of electricity is, and it doesn’t matter what ignites the gas. Once those two innocuous elements come together, they create that “oh” moment for the players.
To avoid making the trap impossible to deal with, it is important to point out the circumstances in a noticeable way. The rogue might notice the oddity of electricity running through the water beside the party for seemingly no reason. Allow your players to make associated Knowledge checks, especially Knowledge (architecture and engineering) or Knowledge (dungeoneering). The concept of water turning into explosive gas is not common knowledge for the average medieval fantasy peasant, but it should still be something a learned individual can know.
The real challenge for the dungeon master running this trap is determining its effects. You can have the explosion deal basically any amount of damage that feels fair to you, but I would scale it as though the players were hit with a fireball spell cast at a caster level equal to the party level. Since you’re treating it like a fireball, give them an appropriate Reflex save for half damage and call it a day if damage is all you want.
For a more dynamic result, treat the explosion as the trigger for something completely separate. It might break through cracks in the floor, walls, or ceiling, revealing hidden compartments, spilling fluid from above or from the sides, or causing the victim to fall into a follow-up trap.
In the end, what you have is a trap that is dynamic from start to finish: its initiation, actual effects, and end result are completely mix-and-match to suit your flavor and intent. I’ve used this particular trap a number of times to great effect, and it pairs nicely with players who are notably wary of obvious dangers because the general setup requires a bit more thought than simply observing the scene.
Put it into play: Use the free D&D 3.5 Trap Generator to establish level-appropriate Search, Disable Device, and save DCs, then replace the generated effect with the chemistry trap above. For more advice on clues, triggers, and fair solutions, read Traps and Why You Need Them.