the fictional logic behind the trap?
There is no fictional logic behind Fourthcore traps, is what I'm saying. The traps exist because they make the game fun and challengeing, not because they "make sense".
(As a side note, the very concept of a trap-filled dungeon doesn't "make sense" to me at all. It's a great game premise, though.)
Look at some of the traps in Fane of the Heresiarch:
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B. Observatory - Glass Dome
If you pry out the wrong gems, you could get sucked into space and instantly killed. Why is this instant death and doesn't give you a chance to catch yourself on something or have another PC grab you? There's no logic here, just "hah hah sucker, you pried out the wrong gems" (because other gems that you pry out give you a
bonus).
B. Observatory - Birth Sign Puzzle
If you touch the wrong sign for your
player's birthday, your
character takes a penalty. The penalties are completely arbitary and are not balanced against each other. There's no logic here.
C. Gauntlet of Walls
This entire trap is completely illogical, yet crazily fun. Why is there a corridor full of walls with completely random and insane traps on them? "Because The Heresiarch built it that way" is one answer, but that's about as satisfying as "because a wizard did it".
For that matter, why is one of the special gems that is the key to The Heresiarch's chamber just resting, untrapped, at the end of this gauntlet of walls? Answer: because in
game terms you need to reward the players for defeating this series of traps. Again, no "fictional logic" here.
Note: the individual wall traps do make a certain amount of sense in that their mechanics more or less line up with "what they do"; i.e., the Sleep Ray Games knock you unconscious, the Disenchanter Saliva-Filled Pit destroys a magic item, etc.
But note that some of these effects are completely outside the bounds of normal 4e mechanics. Unconsciousness is a normal status effect, yes; but the Tomb-Touched Vorpal Razorjack Wall that decapitates you on a crit is not a normal 4e mechanic. (And, why does it decapitate you instead of chopping off an arm or a leg or a hand or a finger? Answer: because this is a funny, game-y thing to do.)
D2. Confessional
Fail the role-playing here and you die, no save. What "fictional logic" does this fit? Why is this Confessional trap instant death, but falling into a room literally full of acid (in the Death Wyvern's Crucible) is
not instant death?[/sblock]
I think that's enough examples to illustrate my point. The traps in Fourthcore are all over the map, and trying to impose logic on them just doesn't work.
If you want to make a game where the traps are Fourthcore-level lethal but have some kind of logic to them, you can do that, but you're going to need to make up your own rules about how to match "logic" with "game mechanics".