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Pathfinder Module: Crypt of the Everflame, pg. 15–16:

Pillar of 1,000 Arrows

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?

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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).

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  • \$\begingroup\$ It really should be mentioned that multiplying by the number of rounds isn’t really fair, since it’s so trivial to avoid the rest of the damage even if it “catches” you initially. \$\endgroup\$ – KRyan Sep 29 '17 at 11:51
  • \$\begingroup\$ multiplying the number of round shouldnt be the total but the average number of rounds people will stay within it's "active zone" without any mean to protect themselves. i'd say like 2.5-3 rounds MAX if they are any good. and it gives a much more reasonable CR \$\endgroup\$ – Mouhgouda Sep 29 '17 at 12:20
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    \$\begingroup\$ Make a kite the size of a tower shield with silk paper, use that to take total cover. +1 \$\endgroup\$ – Mindwin Sep 29 '17 at 13:03
  • \$\begingroup\$ @KRyan Added a paragraph at the end addressing that. \$\endgroup\$ – the dark wanderer Sep 29 '17 at 19:01
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Because it was originally designed as an easy trap

That trap is incredibly dangerous for a party at the level the adventure is designed at (1st). But there are many things to consider on the adventure:

This allows you to heal two points of damage for every point of nonlethal damage taken. One of the pre-generated characters for this adventure is Kyra (cleric 1), which has 4 channel energy uses per day. This is an important fact, as once a character falls unconscious due to nonlethal damage, all nonlethal damage becomes lethal damage, potentially killing the party.

If a creature’s nonlethal damage is equal to his total maximum hit points (not his current hit points), all further nonlethal damage is treated as lethal damage.

At GM discretion, unconscious characters could become immune to the trap, as the trap is not aiming specifically at anyone and was originally designed (by the npc, not the adventure's author) to be loaded with real (lethal) arrows. They would simply wake up at a later time once the damage is naturally healed.

This trap was tailored specifically for this adventure, and there are some things the SRD does not mention:

  • The SRD mentions that a tower shield can bypass the arrows, and anyone behind the tower shield is protected. There are three tower shields as loot a few rooms before, in the same dungeon. The adventure specifically mentions that people use them to bypass the trap, so I wouldn't be surprised if they had some arrows still stuck into them. Holding this cover is a standard action, meaning that they can still move and be completely protected from these arrows.
  • While the arrows are being fired at the group, you can still make Disable Device checks (DC 20) to disable the trap. The pre-generated rogue has a +7 check and has thieve's tools (no penalties). Meaning that on the second round and up, she can try to disable the trap, which considerably lowers the damage output of the trap if she doesn't fail too many checks.
  • The doors leading into the room with this trap are one-way doors, that will shut closed unless someone holds it open, and the trap activates as soon as someone opens the door into the room. This means that the room itself is a bad trap, as you could simply shut the door closed again and avoid it completely, or only one person could protect everyone else by holding a tower shield at the entrance, and so on.
  • The average damage is sketchy here because, with +10 to hit, the trap has less than 50% chance to hit the group's fighter or rogue if they take the Total Cover action (if they decided to not use any tower shield). The total average damage would go down from 100 to 40-50.

With all that said: Yes, the CR for the trap alone is completely broken here. But you should consider where this trap is located as well.

Constructing the trap again

The trap building rules have nothing about nonlethal damage, but we could easily say that doing nonlethal damage reduces the CR by half, as it is hardly a threat. However, if used as written on the SRD, it is way more deadly than what is written on the adventure, especially if the creator designs it to wait for everyone to be inside the room, like triggering when they attempt to open the door to leave the room they are in, or in the middle of a long 10-feet (or 15 feet) wide corridor.

The original trap was designed so you could add any type of arrows inside (the villagers purposedly replaced them by blunt arrows), and thus the nonlethal part could really have been ignored when writing the SRD entry for this trap.

By the construction rules, here is the actual breakdown of this trap's CR if loaded with actual lethal arrows and properly located to maximize the damage output:

  • Perception DC 20: +0
  • Disable DC 20: +0
  • Attack bonus +10: +0
  • Average damage: +20 (4 avg per arrow, rounds to 10, multiply by 2 then by 10 rounds)

It's a CR 2 trap that is active by 10 rounds, making it a CR 20. Yeah, the building rules has a major problem when calculating the average damage, but that is what we got. Personally, I would call it a CR 8 trap based on the Challenge Rating rules for adding multiple monsters into the same encounter.

To build your encounter, simply add creatures, traps, and hazards whose combined XP does not exceed the total XP budget for your encounter.

Being a CR 2 trap, it should give you 600 XP. If we simply add 10 of these traps, that would mean 6000 XP and give us a rounded-down CR 8 encounter.

On the adventure, this trap is simply a fun encounter, not a real trap.

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  • \$\begingroup\$ Can you cite the rule you're referencing and the math? \$\endgroup\$ – Addamere Sep 29 '17 at 13:07
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    \$\begingroup\$ Falling unconscious while the trap's going could mean actual, for-reals death: "If a creature’s nonlethal damage is equal to his total maximum hit points (not his current hit points), all further nonlethal damage is treated as lethal damage." (I think it's possible the trap's author failed to consider this when designing the trap.) \$\endgroup\$ – Hey I Can Chan Sep 29 '17 at 13:07
  • \$\begingroup\$ @ShadowKras I was referring to your statement: "Personally, I would call it a CR 5 trap based on the Challenge Rating rules for adding multiple monsters into the same encounter." What individual CR/XP values are you assigning to what components and combining for a CR 5 encounter? \$\endgroup\$ – Addamere Sep 29 '17 at 13:14
  • \$\begingroup\$ Worse, an unconcious creature is helpless, and thus treated as if they had a Dex mod of -5. I thought that they would be better protected, given that they would fall prone (+4 AC), but this is, alas, not actually the case. \$\endgroup\$ – the dark wanderer Sep 29 '17 at 18:38
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Cr for monsters is not not defined as total damage, unless that damage comes from a single Action that ensures even after death, but rather it is done from damage per round. The same logically holds true for traps. The pillar does not do 200 damage in a single round, but over 10 rounds. It is not likely to instantly disable a level 2 party.

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  • \$\begingroup\$ On Designing Traps: "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 hit more than one target, multiply this value by 2. 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)." \$\endgroup\$ – ShadowKras Sep 29 '17 at 15:17

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