Pathfinder Module: Crypt of the Everflame, pg. 15–16:
Trap: One round after the west door is opened, the trap is sprung and the pillar begins to rotate, firing blunt arrows at every character in the room. The trap runs for 10 full rounds, after which it is exhausted and must be manually reloaded. Each round, the pillar fires 1d4 blunt arrows at each character in the chamber.
Pathfinder Roleplaying Game: Core Rulebook, pg. 423 on Designing a Trap:
Average Damage: If a trap (mechanical or magical) does hit point damage, calculate the average damage for a successful hit and round that value to the nearest multiple of 10. […] If the trap is designed to deal damage over a number of rounds, multiply this value by the number of rounds the trap will be active (or the average number of rounds, if the duration is variable). Use this value to adjust the Challenge Rating of the trap, as indicated on Table 13–3.
The trap above fires 1d4 (AVG=2.5) arrows each dealing 1d8 (AVG=4.5) damage per round at each target, for an average damage per successful hit of 11.25, which we round down to 10. This trap also strikes multiple targets, so we double that to 20, and it runs for 10 rounds so we end up with 200 nonlethal damage. As per Table 13–3, we divide by 10 to arrive at CR 20. That's utter nonsense, and a far cry from the CR 2 rating given in this 2009 module for 1st-level characters.
Where have I gone wrong with the math, and/or what other rules am I not considering? Is this just a mistake on Paizo's part?
Best Answer
1) The trap is incredibly obvious. There's no perception check involved; it's part of the description of the room read as soon as you open the door.
2) The trap is incredibly easy to bypass or disable. You can employ or improvise a tower shield to completely negate its effects, or you could just close the door, or you enter the conveniently placed pit-of-not-getting-hit-by-arrows, or you could run through the room immediately after opening the door, thus getting through the eastern exit well before the trap activated, or you can disable it with a disable device check, or a sheet of paper (objects are immune to non-lethal damage), or a half million other things. In fact, Roldare tells the party that they will "need the shields", if they bother to ask him.
3) Even if a character walks into the room like a moron and stands there long enough to get shot and doesn't use any of the dozens of methods to eliminate the problem, they are unlikely to take any serious damage. The arrows deal nonlethal damage, healed at 1 hp per hour and healed in addition to regular healing if magic is employed. Unless the party is actually slain by the trap, they are gonna just wake up a couple days later. If they do continue to stand in the room for several rounds after the trap activates, yeah, they're probably dead. But if they just stand around for a round or two, they'll probably make it out okay, dragging an unconscious companion or two, and then need a couple hours to recover. And if they have high hp and AC, and especially if they have Deflect Arrows, they might make it through just fine without doing anything competent at all.
In summary, the trap only affects the characters of players who play exceedingly poorly, and even then may not kill them. It's CR 2 because of the beneficial circumstances to the players. The same trap (especially with the DC 20 perception check normally required to spot it, instead of the freebie one in the module) in other circumstances would probably be a somewhat higher CR.
Even then, only if you keep the one-way doors to the room can you reasonably multiply by the number of rounds the trap continues to function, or, rather, multiplying expected per-round damage by maximum duration only makes sense if you expect the trap to affect the players for that long. In this case we should expect the trap to affect the players for no more than one round, since there is no space in the room more than 15' from the conveniently placed pit of safety the townsfolk presumably expect the PCs to employ in case of failure to bring shields, which means the trap deals 20-ish damage and is CR 2 as listed (though with the various other extenuating circumstances the CR should really be lower; not needing a perception check should lower the CR to 1 according to the table in the SRD).