Sit with me for this one. It's quite a story. My only TPK. I believe there is no way I could ever outdo it.
There will be a few Curse of Stradht SPOILERS:
My players met Esmeralda d'Avenir at the Richten's Tower. To give the players a glimpse of what was waiting for them in the Castle, to disclose the location of the silver dragon's skull, and also warn them about that horrible elevator trap wich leads to that part, I had her recount the story of her escape from the Castle. She warned them about the corridor with the elevator trap. She said she felt something was off when she walked it; and later, when she fled back from a band of vampires who ambushed her, she said they didn't follow through this passage. I naïvely thought that would be enough to foreshadow that part of the dungeon and keep them on their guards.
A few sessions later, they're all lv 5. A party of 6. Getting to the castle, they walk this particular corridor like perfect tourists. Nobody cares about what Esmeralda warned them about. Incidentally they walk into that damned elevator trap I tried to avoid having to run. The group is separated. Stradht joins them in the cabin later and pummels everyone to 0HP, except the paladin who panicked and teleported out and fell to a gruesome death down the shaft.
There were 3 players left: a bard, a rogue and a warlock, who didn't get into the trap. I told them that they heard the elevator trap go to the upper most part of the castle, and that if they got to their friends in time, they might be able to save/stabilize them.
They walk the stairs and get to that corridor with the rug and the painting. I was counting on them defeating those easily. I considered the idea of having the vampire lover step on if the encounter proved a bit too easy. I wanted to have them sweat a little to save their friends. But once again, they walked in their ignoring the particularly ominous way I described the place. The bard was attacked by the rug, and the painting started casting spells.
The bard goes first.
Yes : - "The rug is suffocating you and crushing your bones slowly. What do you do?"
Bard: - "I sing a song to help my friend the rogue"
Yes: - "Herr... you might wanna consider, I don't know... trying to force the rug open and get free... I don't know...?"
Bard: - "No way! I only have a strength of 8! It would be a waste of time!"
Yes:- "You know that the sound of your singing might be... muffled with the rug and all!"
Bard: - "I don't care! It's magic, right? So it should work nontheless!"
Yes: - "Rogue, you get one inspiration dice from the desperate song of the bard trapped in the rug. The painting of Stradht waves it's hands and casts a spell on you. You feel it's trying to toy with you mind. Make a wisdom saving throw"
Rogue: -"15!"
Yes: - "You resist it's influence. The spell was "Crown of Madness"' to get you to hurt your comrades. It's your turn. What do you do?"
Rogue: - "I attack the rug with my rapier."
Yes: - "... you know your bard friend is there. There's no way you won't hurt him inside the rug if you attack it with a rapier..."
Rogue: -"He's getting killed anyway! Might as well damage the rug and help him!"
Yes: - "Well... okay. Attack roll."
The rogue attacks the rug, and I tell him there's blood on the rug. I tell him the bard and the rug share the damages. It's the warlock's turn.
Warlock: -"I cast eldritch blast on the painting!"
Yes: - "Ok. Just as a reminder. The painting is just a few steps from you. You have a dagger in your other hand, right?"
Warlock: -" Yeah, I know. I cast Eldritch Blast!"
Yes:-"Okay. You hear an anticlimactic spell fumble sound as the painting uses counterspell on your eldritch blast."
Warlock:- "Seriously?!"
Next turn goes exactly the same way. The bard sings for his rogue friend to continue puncturing him. The rogue attacks the rug. The warlock announces her intention to cast eldritch blast again, except, she fails her saving throw against the painting's "Hypnotic's pattern". It was very difficult for me to see them make all this poor choices and fail all these crucial saving throws but...
I then tell them this. Hoping to shake them a little bit from their catastrophic course of action...
"A young vampire wearing a nightgown opens the door, rubbing his eyelids. He looks at the scene, seeing a tiefling in leather piercing a bloody rug. The rug sings in a half cheering - half painful muffled tone. He eyes the warlock, with her imp on the shoulder, both fascinated by waves of sparkling lights and completely oblivious to the demise of their friends. The young vampire's lips part and he whispers "Yes...". One of his hands goes slowly to his chest and he watches you with intensity... clearly aroused..."
But they didn't change anything.
The bard died in the rug. The rug attacked the rogue. The vampire walked to the bard's corpse and dragged it to his room and closed the door. The rogue died in the rug too, wich in turn killed the warlock. All of their friends in the elevator trap succumbed from their wounds. And the paladin was reduced to a pulp down the shaft...
My first and only TPK to date.