iserith
Magic Wordsmith
The issue then becomes, what happens if their informed decision is to go ahead anyway? That's where the "unfairness" might come in for some players.
Maybe but they'd be wrong.
It's not really an informed decision if the outcome is a TPK if they decide in the opposite direction that the DM expected them to. In other words, by giving them the warning, the DM is trying to remove his own culpability in presenting the "almost impossible" challenge by pushing the responsibility of it to the decision making of the players and ignoring the fact that he is the one who created the situation in the first place.
He is still partially responsible for the TPK. And if he gives warnings that more or less force the players to abandon the idea of going to that encounter, he isn't REALLY giving them a decision. He's kind of railroading the adventure away from that encounter under the fake disguise of allowing the players to decide. Too strong of a warning, the players aren't really getting to decide. Too weak of a warning (or no warning at all), and the DM is setting the group up for a potential TPK.
The responsibility is entirely on the players in this case. They knew the deadliness of the peril (assuming the DM did a good job). They chose to boldly confront it. They lost.
It's not "railroading" to give the players information so they can make choices in the context of the game. If I say "touching the lava kills you," am I railroading you because you decide you don't want to touch the lava? Nope.
It's not a binary choice either - to confront the deadly peril or not. There's at least one other option: Confront the deadly peril at some other time after preparing for it. Getting back to the original topic, that is something that I believe was common to games played with older editions. Gathering info on the threat, preparing in a way that gives you an advantage you didn't have before, and then, perhaps with some luck, seizing victory from the jaws of certain defeat.
Railroading? Fudging? "Metagaming?" C'mon, man, seriously.