Gallery of Lightning Trap

[Nawtyit]

Can someone please explain to he how this trap is only a CR 9? Our GM threw this at us in a homebrew game and I thought the CR was a little low.
I guess it's from Pathfinder Adventure Path #24: "The Final Wish" (Legacy of Fire 6 of 6). As far as I can tell, the CR should be 14 based on +1 per 10 points of average damage (18d6+50%/10=18(5.5)(1.5)/10=148.5/10=14.85 rounded down to 14.

[Are]

I'm guessing the CR is reduced due to it having a "touch" trigger rather than the more common "proximity" trigger of magical traps.

[Strannik]

The average roll for a d6 isn't 5.5, it's 3.5. If you do the math:

18(3.5)(1.5)/10=9.45.

Which is CR 9.

Still, that's a whole lot of damage being thrown around to potentially 18 targets. I think if I were the GM I'd add a point or two just for that.

[Nawtyit]

So, yeah, I can't do math. Thanks Strannik.

[Strannik]

No prob, it happens to all of us sometimes. :)

[Nawtyit]

Follow up question, how is this chain lighting trap a CR 15?
Using the same formula, I think it should be a CR 6.
13d6+50%/10=13(3.5)(1.5)/10=68.25/10=6.822 rounded down to 6.

[njharman]

Challenge Rating of a Trap wrote:

Magic Trap: For a spell trap or magic device trap, the base CR is 1. The highest-level spell used modifies the CR (see CR Modifiers for Magic 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). Use this value to adjust the Challenge Rating of the trap, as indicated on Table: CR Modifiers for Mechanical Traps. Damage from poison does not count toward this value, but extra damage from pit spikes and multiple attacks does.

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

Personally I think the perception and disarm modifiers from mechanical traps maybe should apply to magical traps. But, not RAW. Base is 1, multiple targets 2x damage CR

1 + (18(3.5)(1.5)/10)(2) = CR19 trap

[Strannik]

I forgot about multiplying by 2 for multiple target traps! Thanks njharman!

Yeah, doing that the Chain Lightening Trap is correct: 1+[(13*3.5*1.5)/10]*2=15

The Gallery of Lightening Trap should be:

1+[(18*3.5*1.5)/10]*2=19

Well, looks like I can't do math sometimes too. I told you it happens to all of us Nawtyit! :P

I would bet there was a typo and they left the 1 off in front of the 9 on the CR and then just plugged in the XP for a CR9. Easiest explanation for the mistake.

[Are]

Or they may have thought 19 would be way too high, and instead gave it a CR-value based on spell level.

Of course, the trap was written for 3.5, where chain lightning dealt half damage to the secondary targets, and there was no "multiply by 2"-rule for multiple targets. That may also have played a part in the final CR-value.