Trap CR Issue

[jetpendragon]

One of our DMs has built a his own door and trap. First off its all under water and we are given temporary items of water breathing. The door in the underwater passageway opens after solving a puzzle based from a Greek Mythology tale. If players know the story(out of game) they were able to solve it. If they didn't they had a DC20 Knowledge Religion or DC25 Intelligence check. If they failed the trap would be sprung. The trap would send out projectiles DC20 Reflex. Failure cause the projectile to freeze you solid, and it also has anti magic field incorporated into it. It targets all players, not just the one who springs the trap.
What would the CR for that be? Trying to sort out XP for next game.

[BerronBrightaxe]

Isn't there somewhere a piece of developing own traps and their CR's, in DMG or Dungeonscape?

So the traps springs when 1 of the party doesn't make the check? And then all are entombed in the ice? sounds like a save-or-die trap... the different dc's are high enough for one to fail their save. So I would figure a pretty high CR.

[jetpendragon]

Sorry not quite, one character does the puzzle. Everyone has to dodge a seperate projectile.

[BerronBrightaxe]

Sorry not quite, one character does the puzzle. Everyone has to dodge a seperate projectile.

Ah, so, you get the riddle. You know it, all is golden and one does simply walk into Mordor.. eh, the next room.

If you don't get the riddle, you make a reflexsave. If you make the save, you don't get hit, if you don't make the save you are hit. I presume those getting hit get cold damage (how much?) and as additional effect freeze you solid (no save). Being solid frozen you would start to suffocate, meaning those who haven't have a short while to help their partymembers.

considering you are working with projectile, might I suggest you change the Reflexsave to a ranged touch attack. Those hit get cold damage and have to make an additional fortitude save. If they fail their save, they are frozen, if they make their save, they don't freeze (but still got cold damage).

So multiple targets, damage, saves and a unfavorable setting. I would go with at least a CR of 10 or higher.

[whibla]

Without knowing the party's level (I'm guessing quite low, because of the requirement to give you some means of waterbreathing) it's not possible to give a CR for the trap. However, if the trap affects everyone in the group, and allows a saving throw, then a very simple (house) rule of thumb might be: trap EL = save dc - 11.*

*This kind of assumes the trap is dangerous / lethal, as opposed to being merely irritating. Modify up by a point or two for seriously lethal traps, down if the actual damage or inconvience they cause is minimal. Suffocation rules (for characters intombed in ice), while a tad simplistic, do give a fairly clear indication of how long any untrapped characters have to free their companions before they die. Fun and frantic times.. :P

At the end of the day though, as I said above, encounters, and their xp reward, should really be based around the player's level. While the guidelines on trap cr's are conflicting, the guidelines on xp awards are slightly clearer. Did the pc's expend roughly a quarter of their abilities (spells / hitpoints / whatever) on the encounter. If so, then the el was about their avg. party level. If they used considerably less, then the el was considerably lower. Consider, (as a counter-example to the rule of thumb above) a trap which affects the entire party with a rain of acid, reflex dc 21, causing 20 points of damage on a failed save. Rule of thumb (traps) says el 10, damage dealt by the trap says considerably lower. A 10th level party, while half of them may well fail the saving throw, will brush off 20 hp's without even blinking. In this case I'm not sure I'd even award any xp.

Final note, at the risk of repeating myself, the DMG notes: "You might see such situations as having a CR equal to the level of the party. Simple puzzles and minor encounters should have a CR lower than the party's level, if they're worth an award at all. They should never have a CR higher than the party's level."

[Urpriest]

This depends entirely on what "freeze you solid" means, since that's not a game term. Also, what "has an anti-magic field incorporated" means, since if it's on constantly it will cancel the water breathing and the solid-freezing.

Probably the best basis would be the flesh to ice spell. That's a 5th level spell, so a trap just containing it is CR 5. (Which is enormously silly, but that's trap rules for ya). The DC is three higher than it would normally be, and a different save. Neither of these is a big enough change to alter the CR. There are enough of these to hit every party member simultaneously, so think about it as an encounter with more than one of these things. So for example if you have four party members you're up to EL 9. Then add in whatever you decide the AMF is modeled by.