[RPG] How to correctly run this trap from Curse of Strahd [Minor Spoilers]

curse-of-strahddnd-5etraps

Here is the description for the trap:

This hallway contains an elevator trap, triggered when at least 400 pounds of pressure is applied to the 10-foot square section of floor in the center of the hall… A party of adventurers moving in close formation down the hall is certainly heavy enough to trigger the trap. (Curse of Strahd, p. 74)

Medium-sized creatures occupy a 5 foot square; they cannot occupy the same space per PHB p.191/SRD p.92):

You can move through a nonhostile creature’s space… another creature’s space is difficult terrain for you. Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

Is "close formation" an aspect of the rules I have missed? Given further description of the trap in the text, it seems as if the design of the adventure anticipates more victims than I'd assume:

All creatures inside the elevator (including the unconscious ones) must roll initiative…Each creature inside has one turn to act before the compartment comes to a stop. Their initiative rolls determine the order in which the occupants act…They might search for a way out, wake sleeping party members, cast spells, or take other actions.

How many creatures can be victimized by this trap?

Best Answer

It triggers under the weight of 400 lbs worth of creatures, however many creatures that may be.

There is no "close formation" rule you are missing. The text of the trap is describing characters in close proximity to each other, which is just what it means.

Since the trap is a 10 ft square, using the combat movement rules explicitly as written, 4 Medium creatures could fit in the trap's space. If each creature is about 100 lbs, it's easy to see that they could trigger the trap. Fewer creatures or creatures carrying a lot of stuff could trigger the trap if they still summed to 400 lbs.

However, it is unlikely the text of the trap is trying to be that pedantic. When you as the DM consider that the trap has been laden with the weight of 400 lbs worth of creatures passing over it, you can consider it to be triggered.

For example, you could consider 5 characters marching in a tight cluster to trigger the trap even though the combat movement rules don't technically allow 5 Medium creatures to fit into that space, because in the real world or in a reasonable imagining of the in-game situation it's possible for 5 creatures to fit in a 10 ft square space. Picture any elevator you've ever been in.

Moreover, the combat movement rules consider each character's movement in isolation over a sequence of turns. It's entirely possible that, mechanically, at no point in time do 4 creatures end their turns of movement precisely on 4 distinct 5 ft squares of the trap's space, but that's due to limitations inherent to the combat abstraction. When 4 people are marching in tandem, they're not going one at a time, even if the combat movement rules suggest they are. They're moving simultaneously. So by a reasonable understanding of how people move it's possible for them to all trigger the trap by moving atop it.

Basically, there's no formula or rule to determine when the trap triggers other than when you as the DM estimate that 400 lbs of weight has been applied to it. Depending on how you run your dungeon exploration (square by square, theater of the mind, precise movement, abstract movement, tape measures, fudging it, or so on) you might be more or less picky about it, but it's still up to your judgement, since the actualization of this world is the DM's.