In the next game session, my whole party is almost certainly going to die.
OK, first of all I often told the players (half laughing, half serious), before they start playing with me, that I am a bastart game master that enjoys killing characters and that they should expect frequent TPKs if they do the worng actions.
I believe / think / hope that all game masters do this, so I assume you are along the same line.
Said that, I think there are two kind of TPK:
1) TPK brought by bad planning of the adventure
2) TPK brought by bad roleplaying of the adventure
Number one is your fault -and is bad- while number two is the players' fault -and is very good, as it brings learning and wisdom, things that in life are always useful. If the players will be unhappy with it, just change game or change players: they are not suitable for any kind of gaming and you are not there to plump their egos (unless they pay you suitably, of course!)

Well, that's because they don't know about your second thoughtsThe up side is now, in the next campaign, they now all know that I'm not a toothless DM and I WILL crush the entire party without a second thought.![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.