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The Pathfinder Acid Arrow trap is CR 3, which makes sense, and does "2d4 acid damage for 4 rounds" ... which doesn't make sense (to me at least).

The Acid Arrow spell:

deals 2d4 points of acid damage with no splash damage. For every three caster levels you possess, the acid, unless neutralized, lasts for another round.

Thus a default min-level caster (and aren't all magical effects minimum level unless otherwise specified?) would only only last two rounds, 1 + 3/3 = 2, not four like the trap.

Why is this, and would it be "wrong" to make a CR 2 version that only lasts two rounds? Officially the trap should have a CR based on whichever is higher, either the damage-based CR (base 1 plus 1 for 10 average damage) or the spell level-based CR (base 1 + level 2 spell), but if a CR3 Acid Arrow trap does four rounds of damage, it seems like a trap that only does half that should be a CR lower ... but I don't want to throw an overly difficult trap at my players either.

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    \$\begingroup\$ When you say 'why' do you mean 'what was the dev intent?' or do you mean 'what makes this good?' \$\endgroup\$ – the dark wanderer May 7 '17 at 18:11
  • \$\begingroup\$ When comparing CR, remember that a trap is not going to follow you around or assault you in your sleep like a spellcaster can. A trap is just a dumb stationary thing. \$\endgroup\$ – Ling May 7 '17 at 19:32
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    \$\begingroup\$ "Dev intent" I guess, but not like "how did they decide one feat = 1 AC for Dodge?"' more like "why does this CR 5 monster do X damage when this other CR 5 monster does Y damage?" Traps (and monsters) have rules for creation, but the devs have extra "hidden rules". The exposed rules say a trap uses the higher of damage or level-based CR, and says that magic effects default to min level. But somewhere in the hidden dev part they decided the CR3 acid arrow isn't min level, so I want to understand both why and what a min-level acid arrow trap would be (CR3 also seems wrong). \$\endgroup\$ – machineghost May 7 '17 at 21:31
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    \$\begingroup\$ If it helps anybody puzzle this out, the original D&D 3.5e acid arrow trap is identical except for its expected duration of but 2 rounds. \$\endgroup\$ – Hey I Can Chan May 8 '17 at 0:08
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    \$\begingroup\$ " says that magic effects default to min level. " can you point this reference to me? I see nothing about this on the SRD. \$\endgroup\$ – ShadowKras May 8 '17 at 12:13
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Because the challenge rating would have been the same.

If the trap lasted only two rounds, that means it would cause 2d4 twice (4d4 damage), with a minimum of 4 damage, and an average of 10 damage. That damage is much lower than the average for CR 3 traps, and a lot of level 1 characters could survive being hit by that trap, so it would probably be a CR 1 trap, or CR 2 at most for being magical.

However, due to the fact that the trap is magical, the minimum challenge rating would have been 3 anyway. CR 1 is the base, then for being a spell trap, we increase the CR by +1 per spell level on the trap or per 10 damage caused on average by the trap (the second one doubles if the spell causes damage on multiple targets).

There are far more dangerous CR 3 traps if they decided that it would last two rounds, and that wouldn't be so expensive to craft. Like, a floor with saws will cause 2d6+7 (average 14) and hit multiple targets in a 10 feet area. Comparing both the CR 3 acid arrow trap (6d4) and the CR 3 fireburst statues trap (5d4), you will see that even though the fireburst trap deals less damage (by 1d4, and a low reflex save), it affects a 15-foot cone, so it can potentially affect the entire group of PC's.

Traps do not use the minimum caster level or ability modifier

That is not the only trap that was designed with a caster level higher than the minimum to cast the spell though, as we can see on the Fireburst Statues that uses caster level 5th (CR 3, flaming hands 5d4 damage). While the Burning Hands Trap uses caster level 2nd (CR 2, flaming hands 2d4). A Fireball Trap uses caster level 6th (6d6 damage), as another example.

Taking a second look at the Traps page on the SRD, we see no mention of minimum caster level for traps. So i don't think that's a thing for traps (unlike for magic items), you can use whatever caster level you want, that will simply affect the cost to craft the trap if you are hiring someone to cast the spell for you:

Cost Modifier

+50 gp × caster level × spell level

Unlike magic items aswell, the DC to resist the spell trap effect is also different from the minimum to cast the spell. Normally, for magic items, the DC is 10 + spell level + minimum ability score to cast the spell (ie: 13 int for a firewall). For traps, the DC is based directly on the caster's ability score, so if a wizard with 20 intelligence creates a trap, it will be more dangerous (+1 save DC) than one created by a wizard with 18 intelligence.

Spell Traps: Spell traps produce the spell’s effect. Like all spells, a spell trap that allows a saving throw has a save DC of 10 + spell level + caster’s relevant ability modifier.

Similarly, if a wizard 10 creates his own Fireball Trap, it is far more dangerous than one created by a wizard 6. A fireball cast by a 5th level caster will deal only 5d6 damage (average 17, which rounds to 20, +4 CR), while one cast by a 10th level caster will deal 10d6 damage (average 33, which rounds to 30, + 6 CR). Even though it's a third level spell, the effect could increase the CR by 2 points.

For the Acid Arrow Trap, there was no point in being a CR 2 trap since the spell used is a 2nd level spell (CR +2), which would have the same effects as doubling the damage output of the trap and keep the challenge rating of 3. So, the designer probably tried to obtain the best damage for the challenge rating he was aiming for.

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  • \$\begingroup\$ This is a great explanation, except for a couple things. "Because two rounds would hardly kill anyone." A level 1 character could easily die from an minimum level Acid Arrow trap. Also a trap doesn't have to kill to be a good trap (a CR 1 trap won't kill a level 3, but a corridor lined with several of that trap could). Also, if you did want to build a trap with an effect similar to a min-level acid arrow is that even in the possible in the rules, or do you just get an inferior CR 3 trap (because of the minimum CR thing)? \$\endgroup\$ – machineghost May 8 '17 at 16:20
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    \$\begingroup\$ What level 1 PC character has 4 hit points? Maybe a wizard/sorcerer with -2 con modifier, because otherwise they have 6 hit points at 10 con. \$\endgroup\$ – ShadowKras May 8 '17 at 16:58
  • \$\begingroup\$ My point wasn't to discuss CR, but to explain why the trap is better suited as CR 3 with a 4-round duration instead of 2-round duration. \$\endgroup\$ – ShadowKras May 8 '17 at 17:04
  • \$\begingroup\$ Also, if you did want to build a trap with an effect similar to a min-level acid arrow is that even in the possible in the rules, or do you just get an inferior CR 3 trap (because of the minimum CR thing)? Yes, the spell is CR 3 because the damage is superior to 10, otherwise it would be CR 2 (with 2 round duration). Being magical simply increases the CR by +1, and increases the base DC to discover/disarm by +5. \$\endgroup\$ – ShadowKras May 8 '17 at 17:06
  • \$\begingroup\$ The rules seem to contradict your statement "otherwise it would be CR2". From the SRD: "For a spell trap or magic device trap, the base CR is 1" and "For a magic trap, only one modifier applies to the CR—either the level of the highest-level spell used in the trap, or the average damage figure, whichever is larger." A min-level acid arrow, despite doing half the damage of an official acid arrow trap, would still be CR 3 because (base) 1 + (level) 2 = 3 ... and I still don't understand if this makes sense somehow or whether the rules just fail when it comes to a min level acid arrow. \$\endgroup\$ – machineghost May 8 '17 at 21:10

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