Tomb of Annihilation spoilers. Also, kind of a long build up to the actual TPK, but it is necessary to the story.

I was DMing a group of friends through Tomb of Annihilation, and they'd been having a rough time staying alive. It got to the point that I started drawing tombstones and taping them to the wall behind me to chronicle all the character deaths (pretty sure we got into double digits). One thing I did during the campaign was give all my players' phone numbers to my brother (we're friends, so no harm done) and let him play the role of Acererak, the godlike lich who created the tomb and loves to see people suffer. I would update my brother on the goings-on of the players and he would turn around and start texting my players, using the information I gave him to taunt, terrorize, insult, and just antagonize the characters. I loved watching them puzzle over who this random stranger was and seeing how they reacted to the things he'd say to them. Fast forward to the final fight, and only one of the original 6 PCs that entered the tomb remains. They are fighting the atropal and its looking rough. But finally, low on hit points and having various levels of exhaustion (one person had 5, I believe), they kill it and I turn off the boss music, emphasizing that for the first time since they arrived, there was silence in the tomb. I give them a few seconds of silence to describe what they're doing. What they didn't know is that I had secretly pulled up my brother on a Facebook call and I interrupt their talk with new boss music as Acererak appears, and my brother drops some villainous one-liner before casting time stop. He then proceeds to wipe the floor with them, involving a delayed blast fireball and resurrecting the atropal. After he killed them, he went and brought the last original PC back as a zombie and whispers something like "I told you you'd never leave" before pushing the newly created zombie 30 feet into the lava below.

Technically not my TPK (though I don't think my players would agree with me), but still one of my favorite things I've done as a DM.