Quasqueton said:
You’ve cleared out the dungeon and found the McGuffin you were seeking. Then you come to a room located in the back corner of the dungeon. In the room is only a large lever sticking up out of the floor. You search the room and find a secret door in one wall. You can’t find a way to open the door. The rogue searches the door and lever for traps, and finds none. The monk pulls the lever. He has to make a saving throw – he rolls a 19 on the die, adds in his mods, and fails the save. He turns into a pile of fine dust on the floor.
Okay. Let's see from a player's perspective...
It's fairly obvious that the lever has something to do with the secret door. But, it's also obvious that an
obvious lever that opens a
secret door will not be safe to pull.
So, I'm going to make a few assumptions here...
First, that the Rogue took 20 on his search check to find the opening mechanism of the secret door, and later to find traps on the lever and the secret door. It is likely that he's the best searcher (and certainly the only one qualified to find traps of this sort of difficiulty), and so the traps purposefully have a Search DC that no one in the party can detect.
Second, the Monk will arguably have the best Saving Throws in the entire party. He failed on a 19, which means, metagame-wise, that no one in the party will be able to avoid death with anything short of a natural 20. Again, this is something that can only be purposefully decided and included by the DM.
...that means this entire trap was purposefully designed to be practically unavoidable death. On top of that, it happens after the adventure's victory condition, obtaining the McGuffin, has been satisfied, which only rubs salt in the wound.
Now, how fair this situation is, depends on what a few other circumstances...
Is there a way out other than the secret door? If yes, then it's the Monk's own foolishness that got him killed, since there was no particularly good reason to go through the secret door. If not, then the DM has put the PCs into a no-win situation... Stay put and slowly starve to death, or try the lever and kill yourselves quickly in the hopes of that 1 in 20 chance to survive and escape.
Does the lever actually open the door? If so, then there's a purpose to the death. If not, then the death is completely meaningless.
Anyway, the circumstances will likely determine how "fair" any group thinks the situation is.
In general, I think most people would deem it "unfair" due to the inevitably of the death, the lack of any chance to determine lethalness, the lack of any better options to open the secret door, and the meaninglessness of the death.