Happy Hump Day. :)
I'm trying to design a trap, and I'm having some trouble interpreting the rules. I'm hoping you can help.
The trap in question is a cave with a weakened ceiling. It wasn't constructed by anyone; it's just there. When the PCs enter the area and make a disturbance (e.g. they make a lot of noise), the ceiling will collapse and do damage to them.
Instead of perception checks, like a normal trap, I'd like to call for knowledge (engineering) checks to notice the weakened ceiling.
The trigger for the trap will be noise. I'd like the room to make perception checks vs. the DC of the activities that the characters in the room undertake (as in the "Notice Someone/Something" table in the core rulebook under the Perception skill). Success means the trap is sprung.
The effect will be falling debris, which will do piercing (stalactites) and bludgeoning (rocks) damage to all characters in the room. There will be a Reflex save to halve the damage.
The trap will not reset itself. There is no means of disabling or bypassing the trap.
How do I calculate the CR and XP for this trap? Are there any other features I should consider? The Average Party Level right now is about 9, so I'd like this to be in the Challenging range, around 10.
Thanks,
Chernobyl
Use the traps section of the core rulebook. It's in Environment. However, it only helps if you have an idea for how much damage it deals, how hard the save is, and how hard it is to notice. Guess some numbers, and then use the table to make it work.
There is no means of disabling or bypassing the trap.
Then it's not a trap, but GM fiat. Rocks fall, something happens.
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The room making a perception check to notice feels backwards. If the party notices the weak spot can they make stealth checks to keep quiet? To avoid the opposed roll, you could just say if someone gets a stealth check less than 10 it collapses. As above, the cave in hazard in the core rulebook sounds like what you're after. If you want the CR higher, bumping up the damage by 2d6 and / or raising the save DC by 1 should get you to CR 10.
No way to bypass? If this is the only way to the plot you could have PCs deciding to not risk the cave in and leave. Might work better if it's a faster way through but not the only way. What happens if they use a summoned monster to poke it until it falls?
Is the intention here to trap the party on one side of the debris so they must go forward to get out or just to soften them up before a big fight?
Don't you mean a Cave in hazard?
This. Feel free to scale the damage up/down for different CRs.
The room making a perception check to notice feels backwards. If the party notices the weak spot can they make stealth checks to keep quiet? To avoid the opposed roll, you could just say if someone gets a stealth check less than 10 it collapses. As above, the cave in hazard in the core rulebook sounds like what you're after. If you want the CR higher, bumping up the damage by 2d6 and / or raising the save DC by 1 should get you to CR 10.
Yeah, the cave-in hazard sounds close, except for the trigger ("A character can cause a cave-in by destroying half the pillars holding up the ceiling"). If I swap that for a Stealth- (or Perception-) based trigger, does that affect the CR/XP?
My thinking behind the room having a Perception check was to account for the scenario where the characters enter the room without using Stealth. The room could make a Perception check to "hear" the sound of the party walking (DC 10) or talking (DC 0). It's sort of like the sound trigger of a magic trap, only not as sensitive. If there's a better way to do this, I'm all ears (pun intended).
No way to bypass? If this is the only way to the plot you could have PCs deciding to not risk the cave in and leave. Might work better if it's a faster way through but not the only way. What happens if they use a summoned monster to poke it until it falls?
Is the intention here to trap the party on one side of the debris so they must go forward to get out or just to soften them up before a big fight?
Nope, it's not plot-relevant at all. I'm running AD&D module G2: "Glacial Rift of the Frost Giant Jarl," updated to Pathfinder rules. Area #3 in the upper level is the Empty Ice Cave, which had some peculiar rules governing a cave-in. I'm snapping the whole module to standard Pathfinder rules.
Thanks,
Chernobyl